Trochophora Larvae Cell-Lineages Ciliary Bands

Trochophora Larvae: Cell-Lineages, Ciliary Bands,and Body Regions. 1. Annelida and Mollusca

CLAUS NIELSENn

Zoological Museum (University of Copenhagen), Universitetsparken 15,DK–2100 Copenhagen, Denmark

ABSTRACT The trochophora concept and the literature on cleavage patterns and differentia-tion of ectodermal structures in annelids (‘‘polychaetes’’) and molluscs are reviewed. The earlydevelopment shows some variation within both phyla, and the cephalopods have a highly modifieddevelopment. Nevertheless, there are conspicuous similarities between the early development of thetwo phyla, related to the highly conserved spiral cleavage pattern. Apical and cerebral ganglia havealmost identical origin in the two phyla, and the cell-lineage of the prototroch is identical, except forminor variations between species. The cell-lineage of the metatrochs is almost unknown, but thetelotroch of annelids and the ‘‘telotroch’’ of the gastropod Patella originate from the 2d-cell, as doesthe gastrotroch in the few species which have been studied. The segmented annelid body, i.e. theregion behind the peristome, develops through addition of new ectoderm from a ring of 2d-cells justin front of the telotroch. This whole region is thus derived from 2d-cells. Conversely, the molluscbody is covered by descendants of cells from both the C and D quadrants and a growth zone isnot apparent. This supports the notion that the molluscs are not segmented like the annelids,and that the repeated structures seen in polyplacophorans and monplacophorans do notrepresent a segmentation homologous to that of the annelids. J. Exp. Zool. (Mol. Dev. Evol.)302B: 35–68, 2004. r 2004 Wiley-Liss, Inc.

INTRODUCTION

Ciliated larvae have played an important role inphylogenetic discussions for over a century,especially the larval type called trochophora (seefor example Hatschek, 1891; Nielsen ’79, 2001;Salvini-Plawen ’80; Rouse ’99). Many differentdefinitions of this larval type have been proposed,from the ‘‘classical’’ concept of an ancestral larvaltype or even an ancestral adult type to simply alarva with a prototroch, and I will here review theliterature dealing with cleavage patterns, cell-lineage, ciliary bands, and other ectodermallyderived structures of annelids and molluscs toreach more firmly based conclusions about homo-logies. A similar review on the remaining spiralianphyla is in preparation.

The trochophora concept has been much dis-cussed over the years, with some early as well asmodern authors regarding this larval type ascentral for the understanding of bilaterian phylo-geny (for example, Hatschek, 1891; Jagersten, ’72;Nielsen, ’79, 2001; Gruner, ’80), whereas otherauthors have interpreted it as convergentlyevolved in many groups (Salvini-Plawen, ’80;

Ivanova-Kazas, ’85, ’87; Ivanova-Kazas andIvanov, ’88; Haszprunar et al., ’95).

The first description of a trochophore may beattributed to Loven (1840), who observed anunusually large ciliated larva and followed itthrough metamorphosis to a juvenile worm (thelarva obviously belonged to a species of Polygor-dius, probably P. lacteus, but the adult worm hadnot yet been described). Several subsequentauthors who described planktonic polychaetelarvae referred to this larval type as ‘‘Loven’slarva.’’ Huxley (1853, p 18–19) compared theciliary bands of rotifers and annelid larvae andconcluded that ‘‘the Rotifera are organized uponthe plan of an Annelid larva’’; furthermore heproposed ‘‘that the Rotifera are the permanentforms of Echinoderm larvae.’’ Semper (1872)described the spherical rotifer Trochosphaera,which bears considerable resemblance to a

nCorrespondence to: Claus Nielsen, Zoological Museum (Universityof Copenhagen), Universitetsparken 15, DK–2100 Copenhagen,Denmark. E-mail: [email protected]

Received 5 December 2003; Accepted 5 December 2003Published online in Wiley InterScience (www.interscience.wiley.

com) DOI: 10.1002/jez.b.20001

r 2004 WILEY-LISS, INC.

JOURNAL OF EXPERIMENTAL ZOOLOGY (MOL DEV EVOL) 302B:35–68 (2004)

trochophora larva. Lankester (1874, p 367) intro-duced the term ‘‘trochosphere’’ for larval forms of‘‘worms,’’ echinoderms, and some molluscs, andfor adult rotifers; similar views were expressed, forexample, by Semper (1876) and Balfour (1880).

Hatschek (1878) pointed out that it was confus-ing to use the name of a rotifer for the larval typeand introduced the name ‘‘trochophora.’’ In thatpaper, as well as in his famous textbook(Hatschek, 1891), he gave more detailed descrip-tions of ‘‘the typical trochophore’’ which heconsidered to be the characteristic larva ofZygoneura [¼Protostomia]. His trochophora con-cept was based on several types of larvae,especially the planktotrophic larvae of poly-chaetes, but he considered the embryos/larvae ofthe non-planktotrophic and/or the more or lessdirectly developing annelids as derived. Gastropodand bivalve larvae were likewise interpreted asderived trochophores, and the larvae of turbellar-ians and nemertines were seen as perhaps some-what more ‘‘primitive’’ trochophores. The ciliarybands of rotifers, for example of Trochozoon, madehim regard this group as close to the ancestor of allprotostomes. Also the larvae of the tentaculates(¼lophophorates) were interpreted as derivedtrochophores, although he considered the positionof the whole group as uncertain. Echinoderms(and enteropneusts) were excluded from theProtostomia, and their larvae were not discussed(the textbook remained unfinished). Studies ofcleavage patterns and cell-lineage were still intheir infancy and their phylogenetic importancewas not considered.

In Hatschek’s view, the typical trochophoralarva was a free-swimming, planktotrophic larva(clearly inspired by the morphology of the larvaeof Polygordius and Echiurus) with the followingmain characters: External characters: 1) apicalorgan or ganglion with ciliary tuft; 2) prototroch(‘‘trochus’’, preoral ciliary ring, usually with tworows of cells) engaged in feeding and locomotion;3) adoral ciliary zone engaged in feeding; 4)metatroch (‘‘cingulum’’, postoral ciliary ring)engaged in feeding; 5) gastrotroch (ventral ciliaryband); 6) telotroch (preanal ciliary ring) engagedin locomotion (probably not an original character).Internal characters: 7) gut with oesophagus,stomach, and rectum; 8) nervous system withapical ganglion, paired circumoesophageal nervesto the ventral side behind the mouth, and a pair ofventral nerves (sometimes fused); 9) musclesaround the gut and paired muscles from the apicalorgan to the posterior body; 10) paired protone-

phridia; 11) paired mesoderm originating from apair of teloblasts lateral to the rectum, sometimesdeveloping into longitudinal rows of coelomicsacs. Only very few annelid larvae show all ofthese characters. Larvae of Echiurus have all theciliary bands (Hatschek, 1880) and larvae ofPolygordius have all but the gastrotroch(Hatschek, 1878). Non-planktotrophic larvae gen-erally lack the metatroch (Nielsen, ’98; Rouse,’99), and other of the characteristics listed aboveare modified or even absent in many species,especially in species having direct development.Unexpectedly, planktonic, non-feeding larvae ofsome sabellids have a metatroch and appear tocapture particles, but the particles are transportedtowards the mouth and rejected along the gastro-troch (Pernet, 2003).

In the following, I will discuss characters 1–6, 8,and 10, i.e. the structures which are ectodermal orectodermally derived, in connection with severalold and new observations on cleavage patterns andcell-lineage.

The reviews of the literature on development ofannelids, especially the development of ‘‘poly-chaetes’’ with planktonic larvae, and molluscs willattempt an integration of the results from studieson morphology and cell-lineage, with emphasis oncharacters that can be used in interphyleticcomparisons. Aspects of experimental embryologywill be dealt with only where they can contributeto such comparisons.

ANNELID DEVELOPMENT

Annelid development has been the subject ofmany studies, recently with strong emphasis onleeches, such as Helobdella (see for exampleShankland and Savage, ’97). Both planktotrophicand lecithotrophic larvae and direct developmentare found in a number of ‘‘polychaete’’ families(Nielsen, ’98; Rouse, ’99), but the mainly limnicand terrestrial ‘‘oligochaetes’’ and hirudineanshave direct development.

It should be pointed out that I regard bothechiurans and pogonophorans as annelids (Niel-sen, 2001; see also Hessling, 2002; Hessling andWestheide, 2002; Halanych et al., 2002; Bleidornet al., 2003).

Early development and cell-lineage

The early development of several polychaeteswas studied in great detail about a century ago, forexample Nereis (now Neanthes, Wilson 1892,

C. NIELSEN36

1898), Capitella (Eisig, 1898), Arenicola (Child,’00), Amphitrite and Clymenella (Mead, 1897),Podarke (Treadwell, ’01), Thalassema (Torrey,’03), Polygordius (Woltereck, ’04), Dinophilus(Nelson, ’04), Scoloplos (Delsman, ’16), and re-viewed by Anderson (’66, ’73). (In the followingdiscussion of embryology of these genera, thesereferences will not be repeated.) The larvae ofPolygordius and Thalassema are planktotrophic;those of the others are either lecithotrophic andplanktonic, or they develop without a planktonicstage and are often called direct developing,although they have larval characters, such as aprototroch. There is considerable variation in theamount of yolk in these species, and the cleavagesvary from equal (for example in Podarke andPolygordius, which have small eggs) to highlyunequal (for example in Arenicola and Scoloplos,which have large eggs), but the early pattern isremarkably conserved. Anderson (’73) emphasizedthat differences exist between the fates of specificcells between species, but some of these differ-ences are of a more semantic character. A goodexample is the composition of the prototroch,which is primarily formed on the 16 ‘‘primarytrochoblasts’’ (see below), but where both second-ary and accessory trochoblasts may be involved;the blastomeres are in the same relative positionin all species, and the differences between thespecies are only whether all or only some of thecells become ciliated (and may become deciliatedagain at later stages, although this has not beendocumented in annelids). The spiral cleavagepattern, which has been observed in representa-tives of several phyla, appears highly conserved,and apparently homologous cells can be recog-nized across these phyla. A special nomenclaturefor cell groups and a notation for individualblastomeres have become established, and theseare illustrated in Figs. 1 and 2.

Orientation of the embryo; Regulativepowers

The orientation of the oocyte relative to theovarian epithelium has not attracted much atten-tion, but Lillie (’06) reported that the attachedside of the oocyte becomes the vegetative (blas-toporal) pole of the egg in Chaetopterus. Theprimary, apical-blastoporal axis (related to theradially symmetrical stage) is clearly defined inthe newly spawned eggs in many species, forexample Armandia, Sternaspis, Pectinaria, andPomatoceros (Dorresteijn and Fischer, ’88). The

position of polar bodies indicates the position ofthe apical pole of the embryo, except in specieswhere meiosis is arrested in a stage of the firstdivision awaiting reactivation by fertilization. Itappears that the sperm may enter at any point ofthe egg in most species. The entrance point of thesperm probably determines the plane of the firstcleavage and thereby the secondary, anterior-posterior axis of the embryo (related to thebilateral symmetry), although more detailed stu-dies are badly needed (Morgan and Tyler, ’38;Dorresteijn and Fischer, ’88).

The antero-posterior axis lies through the B andD-cells (and in general through the macromeresoriginating from these cells), and this can easily befollowed through the early ontogeny, for examplein Polygordius and Arenicola (Child, ’00; Wolter-eck, ’04). However, the position of the micromeresis twisted due to the spiral cleavage pattern, sothat the primary trochoblasts 1a2 and 1d2 aresituated at the left side of the embryo and 1b2 and1c2 on the right side (opposite in sinistral species);c- and d-cells occupy the right and left sides of the

Fig. 1. Cell-lineage of Podarke obscura, after Treadwell(’01). Cells of the prototroch in boldface.

CELL-LINEAGES, CILIARY BANDS, BODY REGION OF TROCHOPHORA LARVAE 37

posterior, closing blastopore, respectively (theanterior a- and b-cells enter the stomodealinvagination). The descendants of the 2d-cell, thesomatoblast, and the 4d-cell, the entomesoblast,lie in or near the median plane of the embryo.

Costello (’45) isolated blastomeres of 2– and 4–cell stages of Nereis and observed that the isolatescontinued to differentiate, but the further devel-opment resembled half and quarter embryos andsoon stopped. There was a clear difference be-tween the development of isolated AB and CD cellsand of isolates consisting of AþB, CþD, BþC, andAþD cells, with a partial prototroch developing inall fragments and eye spots and anal pigmentdeveloping only in fragments containing C and Dmaterial. Isolated blastomeres of the 4–cell stagedeveloped prototroch cells but no eye spots. It isclear that axis specification must have taken placebefore the beginning of the cleavage. These, aswell as experiments on later stages summarizedbelow, are in good agreement with early experi-ments on embryos of the polychaetes Lanice(Wilson, ’04b) and Sabellaria (Novikoff, ’38)and on embryos of the oligochaete Tubifex(Penners, ’26).

First two cleavages

The first cleavages show the characteristic spiralpattern (Fig. 3) in which the spindles are not inplane with the primary, apical-blastoporal (ani-mal-vegetal) axis of the embryo and whoseorientation shifts by up to 451 from cleavage tocleavage. The first two cleavages are through theprimary axis and divide the egg into four cells/

quadrants, which define the future axes of theembryo: A (left side of the developing embryo), B(anteroventral), C (right), and D (posterodorsal).Due to the oblique orientation of the spindles, theblastomeres A and C are usually in contact along ashort line at the animal pole, whereas B and D arein contact at a longer line at the blastoporal pole.In eggs with unequal cleavage, the CD-cell andlater the D-cell is the largest, and the orientationof the embryo is thus already fixed at this earlystage. In equally cleaving eggs, the quadrantscannot be distinguished and it has been proposedthat the determination does not take place untila later stage (for example in the gastropodPatella; see below and discussion in Freemanand Lundelius, ’92).

Polar lobes have been observed in only very fewpolychaetes. In Autolytus (Allen, ’64), a small lobewas observed during the first cleavage, andpossibly during the second, and small polar lobeshave also been observed in the ‘‘pogonophorans’’Lamellibrachia and Escarpia by Young et al. (’96).Sabellaria forms large polar lobes at the first twocleavages (Render, ’83).

First micromere quartet (1a–1d)

The third cleavage is usually uneven (Fig. 3), sothe first quartet of smaller apical micromeres,1a–1d, can easily be recognized. Cell-lineagestudies have shown that their descendants formhead structures including the apical rosette cellswith the apical organ (cells 1a111–1d111), headectoderm, and the primary trochoblasts (1a2–1d2).In the following, the term episphere will be used

2c2

1c12

1b211b22

1b111b12

1a11

1a21

1a22

1a12

1d11

1d21

1d22 1d12

1c11

1c21

1c22

2c1

2d2

2d1

2a2

2a1

2b1

3B

3A3C3c

3d

3a

3b

4D

4d

2b2

B-quadrant (yellow)B-quadrant (yellow)

A-quadrant (blue)

A-quadrant(blue)

C-quadrant(green)

D-quadrant (red)D-quadrant (red)

Fig. 2. Diagram of a 33–cell stage of an unequally cleavingspiral-type embryo (based on Arenicola; Child ’00). The fourquadrants, i.e. the descendants of each of the four firstblastomeres, are separated by heavy lines (the quadrants havethe indicated colors in Figs. 3–6 and 7–9). The cells of the first

micromere quartet are white, those of the second quartet lightgrey, those of the third quartet black, and the macromeresand the 4d-cell are white. The notation for the blastomeres, ascan be followed in Fig. 1, is indicated.

C. NIELSEN38

for the ‘‘upper’’ part of the larva covered by thecells of the first micromere quartet (plus thesecondary trochoblasts when present, see below).The apical organ is discussed separately below. InArenicola it was found that the cells 1c112112 and

1d112112 are thicker than the neighbouring epithe-lial cells and that they divide perpendicular to thesurface so that the ectoderm becomes two-layered;these cells were interpreted as the primordia ofthe cephalic ganglion (see below).

Fig. 3. Cleavage of Arenicola marina, based on Child (’00).In this and all the following diagrams the quadrants have thefollowing colors: A: blue, B: yellow, C: green, D: red. In C-Othe 2d-cell and its descendants are dark red; in C-E and N-Oother D-quadrant cells are pink. The primary prototroch cellsare cross-hatched in C-J, the secondary prototroch cells arehatched in D, and the endoderm is grey in F-N. The lineage ofthe telotroch cells can be followed by the dots on 2d-cells in E-P. The cells marked with a cross in K, L, and O will fuse in theventral midline in front of the presumptive anus. A-D seenfrom the apical pole; E-L seen from the posterior side; M is a

right view; and N-P are blastoporal views. A, 4–cell stage. B,8–cell stage. C, 16–cell stage. D, Stage with 19 primary andnine secondary prototroch cells. E, The 2d-cell has dividedonce. F, Second division of the 2d-cell. G-K, Further divisionsof the 2d-cells leading to the formation of a line of six telotrochcells. L, the line of telotroch cells curves ventrally, surround-ing the future anal area. M, Stage close to K in right view. N,Early gastrula. O, late gastrula; the elongate blastopore isbordered by cells of all four quadrants, but the A and B-quadrant cells occupy only a small anterior area. P, Earlystomodaeum, the telotroch is now almost fused ventrally.


The primary trochoblasts divide only twice(Fig. 3). This appears to be the general patternin polychaetes. The development of prototrochcilia has not been followed in all the species, but itwas reported that all 16 cells develop cilia inAmphitrite and Thalassema and Scoloplos. How-ever, in Nereis and Clymenella only three of thedescendants of the primary trochoblast cellsdevelop cilia, viz. the cells 1m211, 1m212, and1m222. Accessory trochoblasts (1a1222–1c1222) areformed, for example in Amphitrite and Podarke,but not in Arenicola (see also Damen and Dictus,’94). The descendants of the 1d12–cell occupy theposterior gap in the prototroch in early stages andspread further posteriorly to form the antero-median part of the dorsal hyposphere, for examplein Amphitrite, Arenicola, and Thalassema. At alater stage the primary prototroch cells of the Cand D quadrants move together to form acomplete ring, cutting the just mentioned cellsoff from the episphere. Wilson (1892) with quitesome reservation interpreted the two elongatecells 1c11221 and 1d11221 situated under the proto-troch cells as nephroblasts (protonephridia) or‘‘head-kidneys,’’ but this seems to be a misunder-standing, and cells in a similar position becomemucous glands in Amphitrite.

Isolated micromeres of the 8–cell stage ofNereis developed into small spheres, differentiat-ing as if they were in place in a whole embryo.Isolated macromeres developed into non-ciliatedpartial gastrulae with an endodermal cellpartially surrounded by ectodermal cells. Isolatedmicromere quartets developed cilia, whereasno cilia was ever observed on partial embryosdeveloping from the blastoporal quartet(Costello, ’45).

The concept of an annelid cross, versus amolluscan cross, has haunted the phylogeneticliterature for decades (Scheltema, ’93; Rouse, ’99;Rouse and Fauchald, ’95), but most of theseauthors just include the character into their datamatrices without any discussion of the value of thecharacter. Other authors, both early and recent,question the phylogenetic value of the character(Child, ’00; Salvini-Plawen, ’85; Nielsen, 2001;Jenner, 2003) or completely disregard it (Ander-son, ’73). The spiralian cleavage results in a ratherinvariant blastomere pattern in the first quartet,but some species have certain groups of cells ofspecial shapes or sizes, so that for example a crosspattern becomes obvious for example in themolluscs Stenoplax and Lymnaea (Heath, 1899;Verdonk and van den Biggelaar, ’83). However, if

the shadings are removed from the drawingspresented by the various authors, it becomes verydifficult or completely impossible to recognize thecross patterns in most species, therefore theconcept of the cross should be eliminated fromphylogenetic discussions.

Second micromere quartet (2a–2d)

The 2a–2c-cells mainly develop into the stomo-daeum, probably the adoral ciliary zone (whenpresent, see below), and to a usually rather smallpart of the anterolateral ectoderm, but some of thedescendants of 2a1–2c1 develop into secondarytrochoblasts. In Nereis, Amphitrite, Arenicola(Fig. 3), and Clymenella, nine secondary trocho-blasts are formed, viz. the cells 2a111–2c111, 2a112–2c112 and 2a121–2c121; in Podarke, only six werereported, 2a112–2c112 and 2a121–2c121; and inScoloplos, which has a non-pelagic larva, onlytwo small secondary trochoblasts, 2a1 and 2c1,were found (see also Damen and Dictus, ’94). Inthese species, the secondary prototroch cells fillthe gaps between the primary prototroch cells,except at the posterior side (Fig. 3).

The 2d-cell (or one of its descendants) is thesomatoblast, which is almost median in positionand very large, for example in Arenicola andPlatynereis (Dorresteijn, ’90), but rather small forexample in Podarke. It divides profusely, givingrise to the main part of the body ectoderm byextending laterally to meet in the ventral midline,enclosing the posterior part of the blastopore (seebelow). The longitudinal extension of the ecto-derm during the addition of segments from theposterior zone takes place through cell division ina narrow posterior growth zone of 2d-cell descen-dants (just anterior to the telotroch when present;see below). In polychaetes, large, well-definedectoteloblasts have not been observed (althoughthe word is often used), but several clitellates havea ring of conspicuous ectoteloblasts, which give offlongitudinal bands of much smaller cells (see forexample Dohle, ’99; Shankland and Savage, ’97).In leeches, there are four ectoteloblasts on eachside, and these eight cells are descendants of the2d-cell, also called MNOP. The paired ventralnerves with ganglia and peripheral nerves developfrom these cells (Torrence and Stuart ’86).

In Arenicola, the telotroch (called paratroch inmost of the older literature) develops from cells ofthe D-quadrant, viz., from a slightly curved line ofsix cells, descendants of the cells 2d1222 and 2d2121;the line curves further, becoming horseshoe-

C. NIELSEN40

shaped and finally closes as a circle surroundingthe cells of the posterior edge of the fusedblastopore (2d-cells), where the anus will soonbreak through (see Fig. 3 and below). The embryosof Amphitrite show four telotroch cells, apparentlyof the same origin and similarly surrounding 2d-cells of the proctodaeum (see also Robert, ’03).Developmental stages of Capitella show a ring of ahigher number of telotroch cells, which should bederived from the somatic plate.

Most of the cell-lineage studies have treatedlecithotrophic larvae, which lack metatroch and awell-defined adoral ciliary zone, but the larva ofPolygordius is planktotrophic. Here it was re-ported that the anterior descendants of the cells3c1 and 3d1 should become incorporated in thelower lip, whereas their posterior sister cellsshould form the main part of the metatroch.However, it seems more likely that the ciliatedposterior cells, which according to the drawingform posterolateral extensions of the ciliated lowerlip, represent the lateral parts of the adoral ciliary

zone. The metatroch could then originate on theneighbouring cells 2d2112 and 2d122. The diagram(Fig. 4) is color-coded in accordance with thisinterpretation.

These observations of the origin of the meta-troch (as interpreted here) and telotroch are in fullagreement with the trochaea theory (Nielsen, ’79,2001) which proposes that proto-, meta-, andtelotroch represent segments of the circumblasto-poral archaeotroch in the early ancestral proto-stome, the trochaea.

Third micromere quartet (3a–3d)

Cells of the third quartet are usually inconspic-uous. The posterior descendants of 3c1 and 3d1

have been reported to form the major part of themetatroch in Polygordius, but this interpretationseems questionable, and it appears more likelythat they form the lateral parts of the adoralciliary zone (see above). Descendants of the cells3a2–3d2 surround the blastopore and form the

Fig. 4. Development of the hyposphere of Polygordius,based on Woltereck (‘04). Colors of the quadrants: A: blue, B:yellow, C: green, D: red. In F, the descendants of the 2d-cellare red and the other D-cells pink. The ciliated cells

interpreted as metatrochal by Woltereck are indicated by finearrows; they are here interpreted as belonging to the adoralciliary zone, whereas the nuclei of the cells here interpreted asmetatrochal are indicated by fat arrows.


major part of the stomodaeum (in Arenicola) andsmall areas lateral to the mouth. Other cells movein from the blastoderm and become ectomesoderm(larval mesoderm). In Scoloplos, the ectomeso-derm originates from all four quadrants, whereasonly the cells 3a222, 3c212, and 3d222 have this fatein Podarke. In Polygordius, descendants of thecells 3c2 and 3d2 develop into protonephridia andendomesoderm. Several authors have claimed thatthe ectomesoderm gives rise to ‘‘larval muscula-ture,’’ but this seems not to have been convin-cingly demonstrated. Treadwell (’01, p 426) statedthat some descendants of the 3c-cell may partici-pate in the formation of the proctodaeumin Podarke, but this is not supported by thedrawings.

Fourth micromere quartet (4a–4d)

The cells 4a–4d become endoderm and invagi-nate soon after the macromeres. In Amphitrite andArenicola, the 4d-cell is large and divides into twolateral cells shortly after the 64–cell stage. Thesetwo cells begin to extend into the narrow blas-tocoel, but remain connected to the surface of theembryo for some time while the inner part of thecells increases in size. Finally, just before gastru-lation begins, they move completely into theblastocoel. At gastrulation, the mesoblasts there-fore become situated at the postero-dorsal side ofthe blastopore. However, in Polygordius, the 4d-cell descendants remain in the thin blastodermuntil after gastrulation and formation of stomo-daeum and anus, and become situated lateral oreven ventral to the anus (Fig. 4). The differentia-tion of the mesoderm will not be discussed here.

Macromeres (4A–4D)

The macromeres become situated inside theembryo and become endoderm (midgut). Theendoderm closes around a gut cavity in the fewspecies having planktotrophic larvae (for examplePolygordius), and mouth and anus originate fromstomodeal and proctodeal invaginations, respec-tively. In non-planktotrophic species, a gut lumenis narrow or absent, and species with epibolicgastrulation have a compact endoderm (for exam-ple Nereis).

Gastrulation, blastopore closure, andfurther growth of the ectoderm

Many species with small eggs have embolicgastrulation, which makes it easy to follow thefate of the blastopore. Larger eggs with much yolk

usually go through epibolic gastrulation, so astraightforward blastopore, and hence blastoporeclosure, is more difficult to observe. However,species with large amounts of yolk, such as Nereis(Wilson, 1892) and Tomopteris (Akesson, ’62)show cell lineages almost identical to those ofspecies with small eggs, only the relative sizes ofthe blastomeres are different.

The general picture is that the blastoporebecomes longitudinally elongated and laterallycompressed (Figs 2, 3). The lateral blastopore lips,consisting of cells of the 3rd micromere quartet,fuse in the midline. Anteriorly, a part of theblastopore usually remains, but this region movesinto the embryo through the formation of a largestomodaeum, consisting mainly of cells from thethird quartet. Posteriorly, the blastopore finallycloses completely, surrounded by 2d-cell descen-dants, but a proctodaeum soon develops in thesame area. During the following development, aring of 2d-cells around the proctodaeal area, justin front of the telotroch when present, becomesthe ectodermal growth zone. The ectodermal partof the postoral segments, i.e., the whole body ofthe worm except prostomium and peristomium, isthus formed from the cells of the D-quadrant, ormore specifically from the 2d-cells of the somaticplate. In Thalassema, the blastopore is very shortand after the ingression of the 4d-cell the posteriorcells of the somatic plate should migrate aroundthe posterior pole of the embryo and forwards toform part of the lower lip. However, this was notdocumented, and it cannot be excluded that alateral blastopore closure had actually taken somepart in the apparent expansion of the 2d cell plate.

The classical cell-lineage studies indicate that inthe adult annelid, the epithelium of prostomiumand peristomium consists of cells from all fourquadrants, whereas the whole segmented body iscovered by descendants of the 2d-cell originatingfrom a posterior, circumanal growth zone (Fig. 5).This is in full accordance the results obtainedthrough experiments with markings of singleblastomeres of 4–cell stages of Chaetopterus(Henry and Martindale, ’87; see Fig. 6). It alsoagrees with the well-known cell-lineages of severalclitellates (Dohle, ’99; Shankland and Savage, ’97).Anderson, (’73, p. 18) stated that descendants ofthe 3c and 3d-cells are included in the growth zonein certain species, but this may be due to amisinterpretation. It therefore seems more appro-priate to describe the B and D quadrants asanterior and posterior, respectively, than asventral and dorsal as often seen, for example in

C. NIELSEN42

studies of mollusc development. Shankland andSeaver (2000) pointed out that micromere quar-tets 2 and 4, as mentioned above, are rotatedrelative to the antero-posterior axis, so that forexample the episphere has a pattern with a- and d-cells on the left side and b- and c-cells on the right.

However, the 2d- and 4d-cells give rise to majorparts of the adult body, so it seems natural to usethese cells as markers for the orientation.

Ciliary bands

The ciliary bands on both larval and adultannelids are formed from multiciliate cells exceptin Owenia, which has only monociliate cells (seebelow).

A conspicuous prototroch consisting of com-pound cilia is present in most polychaetes with aplanktonic development (Nielsen ’87; Pernet et al.,2002), but a wider band of shorter, probablyseparate, cilia has been observed for example inAmphitrite and Clymenella (Mead, 1897) andArenicola (Child, ’00). The cell-lineage of thesetwo types of ciliary bands is identical. The 1–setiger larval stage of the direct developingMarphysa is almost spherical with a prototrochof separate cilia covering most of the body and asmall telotroch (Pernet et al., 2002). On thecontrary, the direct developing Capitella has anelongate larva with small and narrow prototrochand telotroch (Eisig, 1898). A completely ciliatedblastula stage, consisting of an unusually highnumber of cells, has been observed in Mercierellaand Polydora (Rullier, ’55, ’60). Later stages arenormal trochophores with non-ciliated zones andprototroch (plus telotroch in Polydora). The cell-lineage of this type of development is unknown. Itappears that only larvae with both a prototroch

apical tuft

episphere

prototroch

metatroch

gastrotroch

telotroch

anus

protonephridium

prostomium

peristomium

mouth

anus

segmented body

growth zone

pygidium

adoral ciliary zone

Fig. 5. Interpretation of the contribution of the four quadrants to larval and adult polychaete body.

dorsal ventral

A C

D

B

Fig. 6. Contributions of the four quadrants to theectoderm of the juvenile Chaetopterus sp., based on Henryand Martindale (’87).


and a metatroch of compound cilia (and the larvaof Owenia) are ciliary filter-feeders. My ownunpublished observations on the development ofSerpula columbiana (from Friday Harbor, WA)show that the prototroch cells initially developseparate cilia which subsequently become orga-nized in compound cilia. Normal deciliation ofcertain prototroch cells, as described from Patella(see below), has not been observed, but it is not tobe expected that all blastomeres are either ciliatedor non-ciliated, or have separate or compound ciliaduring the whole ontogeny. There is a ring muscleand a ring nerve along the base of the prototrochand/or along the metatroch (Woltereck, ’02;Lacalli, ’84; Hay-Schmidt, ’95; Voronezhskayaet al., 2002). The origin of both types of cells isunknown.

Prototroch

The prototroch is the most conspicuous ciliaryband of polychaete larvae and develops from thetrochoblasts (primary, secondary, accessory; seeabove). There is usually only one ring of cells withcompound cilia, but the prototroch of Polygordius,and perhaps also of Arenicola (Child, ’00), consistsof two rings of cells with compound cilia, withabout 64 cells in each ring (Hatschek, 1878;Fraipont, 1887).

The primary trochoblasts initially form fourgroups, two anterior and two posterior. The gapsbetween these cell groups may become filled bysecondary trochoblasts and in some species also byaccessory trochoblasts of the quadrants A-C. Thus,initially there is always a gap in the prototrochbetween the primary trochoblasts of the C and Dquadrants, but the cells of the two groups usuallymerge dorsally at a later stage forming a completering. Compound cilia consisting of many cilia caneasily be recognized with the light microscope, butit is difficult to distinguish compound cilia con-sisting of only few cilia from separate cilia; thismakes it difficult to interpret many of the olderreports. Owenia has only monociliate cells withseparate cilia (Nielsen ’87; Emlet and Strath-mann, ’94), but since the oweniids are highlyspecialized polychaetes (Rouse and Fauchald, ’97)it is very unlikely that the monociliate condition inone genus of this family should be an annelidplesiomorphy.

Adoral ciliary zone

Planktotrophic polychaete larvae have a band ofmulticiliate cells with separate cilia situated

between prototroch and metatroch and surround-ing the mouth. These cells apparently convey foodparticles captured by prototroch and metatroch tothe mouth. Here they may be ingested or rejected,and the rejected particles may be carried awayalong the gastrotroch (see below). Woltereck (’04)observed that three cell groups on each side,descendants of 3c1 and 3d2, form the lower lip ofthe mouth, which is at the central part of theadoral ciliary zone. He reported that the two cells3c1post and 3d1post should form part of themetatroch, but since these cells are depictedwith cilia at the edge of the mouth (Woltereck’04, fig. 11) it appears more likely that these ciliaare in fact part of the adoral ciliary zone; themetatroch could then develop from the neighbour-ing cells 2d2112 and 2d122 (see Fig. 4).

Metatroch

Planktotrophic larvae have a metatroch which isa postoral, peristomial band of compound ciliafunctioning in a downstream-collecting systemtogether with the prototroch and the adoral ciliaryzone (Nielsen, ’87; Rouse, ’99; Riisgard et al.,2000). A number of sabellid larvae have ametatroch and may capture particles, but mouthand gut are not fully developed and the particlesare ‘‘rejected’’ at the mouth area and carried awayalong the gastrotroch (see above). The compoundnature of the cilia can generally not be ascertainedwithout the use of electron microscopical techni-ques, and has only been documented for membersof the families Serpulidae (Serpula, Nielsen, ’87),Polygordiidae (Polygordius, Nielsen, ’87), Ophelii-dae (Armandia, Miner et al., ’99), and Capitellidae(Notomastus, Pernet et al., 2002). The cell-lineagehas only been studied in Polygordius (see above;Fig. 4). Interestingly, the 3–segmented metatro-chophore of Platynereis, which has no metatroch,shows engrailed-protein staining not only of theciliated cells of the prototroch and the threesegments, but also in a stripe along the posteriorside of the peristomium, in the place where themetatroch is situated in the planktotrophic larvae(Dorresteijn et al., ’93). Other postoral ciliarybands of compound or separate cilia are moredifficult to interpret, but most of them aredefinitely not metatrochs (see below).

Gastrotroch

The gastrotroch (neurotroch) is a longitudinalposterior extension of the adoral ciliary zonepassing through ventral breaks in metatroch and

C. NIELSEN44

telotroch, when present (see above and below). Itconsists of multiciliate cells with separate cilia. InScoloplos, it is first recognized as a double row ofciliated cells extending from the stomodaeum tothe telotroch in a late larval stage; its cell-lineageis not known, but it is believed to develop from thelateral margins of the fusing lateral blastopore lips(2d-cells). The early study of Capitella doesperhaps support this supposition. In plankto-trophic larvae, it apparently functions as a rejec-tion tract for unaccepted particles caught by thedownstream-collecting system and transported tothe mouth by the adoral cilia, but in other speciesit may function as a creeping sole at metamor-phosis. Many smaller polychaetes, for examplemany interstitial species, retain the ciliatedventral zone along the ventral nerve cord(s).

Telotroch

A telotroch, i.e. a perianal ring of cells carrying arow of compound cilia (Nielsen, ’87), is found inrepresentatives of several polychaete families,both in species with planktonic larvae and thosewith direct development. The lineage of these cellsis known only from Arenicola and Amphitrite (seeabove), but several of the classical embryologicalstudies report the presence of a row of four or sixcells which curve around the anal region and formthe telotroch. The telotroch is interrupted ven-trally, for example in Polygordius and someterebellid larva (Thorson, ’46; Nielsen, 2001)where the gastrotroch passes towards the anus.Full-grown lecithotrophic larvae with a telotroch,such as Arenicola, Nereis, Amphitrite, Pectinaria,many spionids, and Lamellibrachia, have a ringof compound cilia that is complete ventrally(although not always dorsally)(Thorson, ’46;Nielsen, ’87; Pernet et al., 2002).

Additional ciliary rings

Several polychaete larvae have more or lesscomplete ciliary rings between prototroch andapical organ. They have been called acrotroch ormeniscotroch (Rouse, ’99), but their cell-lineagehas not been studied, and their homologies seemuncertain.

Many species have a ring of cells with a line ofshort, separate cilia, called a pretroch, along theapical side of the prototroch, but the lineage ofthese cells has not been studied. Spirobranchusand Phyllodoce have a pretroch, a ring of largeprototroch cells with compound cilia, and two rowsof smaller cells with long, separate cilia behind the

large ring (Lacalli, ’84, ’86); this may be the morecommon pattern in planktotrophic larvae (Heim-ler, ’88). Wilson (1892) reported the presence of aring of smaller ciliated cells in the position of thepretroch in Nereis, but their lineage was notdescribed. In Amphitrite, the prototroch is formedfrom the 16 primary trochoblasts plus the ninesecondary trochoblasts. Keeping in mind that bothaccessory and secondary trochoblasts contribute tothe prototroch in the gastropod Patella and thatciliated primary trochoblasts may become deci-liated during prototroch differentiation (seeDamen and Dictus, ’94) it seems impossible todraw conclusions about cell-lineage of the variousrings of ciliated cells in annelids without directobservations on cell-lineage.

The larva of Eunice (Akesson, ’67) shows a bandof cilia behind the prototroch, and this has beeninterpreted as a metatroch, for example by Rouse(’99). However, it consists of separate cilia and iscontinuous with the gastrotroch. It seems morelikely that it represents the adoral ciliary zone.

Many polychaete larvae develop ciliary bands orrings of separate or compound cilia on thesegments in the metatrochophora stages, forexample spionids, nereids, and sabellariids(Hannerz, ’56; Pernet et al., 2002). Chaetopteridlarvae develop two rings of compound cilia aroundthe equator at an early planktonic stage (Enders,’09; Pernet et al., 2002). Rouse (’99) interpretedthe ring that develops first as a metatroch and thesecond as a telotroch, but later ontogenetic stagesshow these bands situated between the posteriorsegment of the ‘‘forebody’’ and the segment withthe wing-shaped parapodia, i.e. at around segment9–10 (Irvine et al. ’99), so they are clearly notsituated on the peristomium as a metatroch.

Nervous system

The nervous system of most annelid larvaecomprises an apical organ/complex, with apicalganglion and paired cerebral ganglia, connected tothe ventral nervous system through a pair ofcircumoesophageal connectives. The apical com-plex is well-developed in the larvae, whereas therow of paired ganglia found in many laterpolychaete larvae is associated with the ‘‘adult’’segments.

Apical organ and adult brain

The cell-lineage studies discussed above gener-ally state that the cells 1a111–1d111 become theapical ganglion, but there seem to be no direct


observations to support this assumption. Theapical organ bears a tuft of cilia in many younglarvae, but the tuft disappears in later stages; itsfunction is unknown.

In meticulous TEM studies of 24– and 48–hourtrochophores of Spirobranchus, Lacalli (’81, ’84)documented 16 cells in the apical organ, withrather small differences between the two stages.Only four of the 16 cells are exposed on thesurface, one carrying a long apical tuft and onecarrying a smaller tuft of shorter cilia, and twowith a narrow surface without cilia. Other cellscomprise three cells with apical, digitate processeswhich form an internal ‘‘apical plexus’’ (later onsupplemented by two additional apical cells) and anumber of surrounding capsular cells. Axons fromtwo bipolar nerve cells enter the right side of theapical organ and make contact with the apicalplexus. One of these has a cell body at the ventralside of the episphere near the midline and has aneurite extending to the prototroch and along theprototroch cells as the prototroch nerve. The othercell is the sensory cell of the eye spot and sends aneurite to the apical organ and one to the neuriteof the first-mentioned cell and further to theprototroch nerve. Two further cells are associatedwith the prototroch nerve. Other nerve cells areassociated with a metatroch nerve (incompletemid-ventrally), a ring nerve around the pharynxwith a pharyngeal complex and a suboral complex,and a small gastrotroch nerve extending towardsthe anus.

The young, apparently lecithotrophic trocho-phores of Phyllodoce (Lacalli, ’81) show a nervoussystem resembling that of Spirobranchus, but theapical tuft is formed from three cells and there arepaired eye spots and apical nerves.

The metatrochophore of Spirobranchus (Lacalli,’81, ’84) has developed an eye spot on the left side(a second one develops soon after), and an extraeye spot close to the first one on the right side,which have all been incorporated into pairedcerebral ganglia. The cell-lineage study of Areni-cola (Child, ’00) indicated that the cerebral gangliacould develop from two cells (1d112112 and 1c112112)adjacent to the apical cells, but this should bechecked with modern methods. The ganglia areconnected through a prominent commissure,together forming a dumbbell-shaped brain. Theapical organ has fused with the commissure, andthe apical ciliary tuft has been lost. The two apicalnerves can still be recognized but the main nervesare now a pair of lateral nerves from the cerebralganglia passing the prototroch, apparently with-

out connecting to it and the metatroch, andconverging to form a pair of ventral nerves withcommissures.

In an immunocytochemical study of early andlate metatrochophores and early juveniles ofPolygordius, Hay-Schmidt (’95) observed nervecells in a pattern consistent with the nervoussystem described above. Three serotonergic cellbodies were observed in the apical organ at bothstages and two small groups in a suboesophagealganglion. FMRFamide-positive cell bodies arescattered over the apical organ and paired groupsdevelop in the region of the suboesophageal gang-lion.

A pair of ‘‘pioneer’’ nerve cells at the pygidiumwith axons extending forwards to the prototrochhas been observed in 48–hours metatrochophoresof Platynereis (Dorresteijn et al., ’93) and a singlesuch cell has been observed in an early stage ofPhyllodoce (Voronezhskaya et al., 2003), and alsoin early larvae of Serpula oregonensis (by Dr.Stephen Kempf and the author). The significanceof these cells is unknown.

The whole larval nervous system is intra- orbasiepithelial.

The results of light microscopical investigationsof the nervous system of whole larvae (for exampleon Polygordius, Woltereck, ’02), and of histologicalsections (for example on Pectinaria, Korn, ’60) aswell as modern immunocytochemical studies (forexample on Echiurus, Hessling, ’02; Hessling andWestheide, ’02) are in good accordance with theTEM studies.

Segmental ventral ganglia begin to develop atthe metatrochophora stage.

Eyes

Most annelid trochophores have one, two, ormore pigmented eyes on the episphere. Their cellsare modified epidermal cells, which sink downfrom the epithelium and become covered almostcompletely by surrounding epithelial cells (Mars-den and Hsieh, ’87; Bartolomaeus, ’92a). Axonsconnecting the eyes with both the apical organ andthe prototroch have been documented in a fewspecies (Lacalli, ’81, ’84; Marsden and Hsieh, ’87).

Bartolomaeus (’92a) reviewed the ultrastruc-ture of the eyes and found that all speciesinvestigated have 1–2 rhabdomeric sensory cells(some with a short cilium and some with adiplosome) and 1–2 pigment cells in each eye;Autolytus and Lanice have an additional sensorycell with a slender process containing a terminal

C. NIELSEN46

centriole. An additional, supporting cell wasreported in Capitella by Rhode (’93). The rhabdo-meres are facing the pigment cell (inverse eyes)oriented parallel to almost perpendicular to theincoming light; in Autolytus, the pigment cellsecretes a small lens. The lineage of the cells of theeyes has not been documented, but the observa-tions on regeneration of Nereis embryos (Costello,’45) indicate that they originate from cells of theC- and D-quadrants.

Larval eyes are lost at metamorphosis in somespecies, for example Nereis, Platynereis, andCapitella (Eakin and Westfall, ’64; Rhode, ’92,’93; Arendt et al., 2002), but transformed intoadult eyes in others, for example in opheliids(Wilson, ’39; Bartolomaeus, ’93). In Platynereis(Arendt et al., 2002) the adult eyes develop in themetatrochophora stage when the larval eyes arestill present, so confusion between the two types inother species cannot be ruled out.

Other putative eyes, including the so-calledphaosomes, are found in a few polychaetes (Eakinand Hermans, ’88), but there is not enoughinformation to use such structures for compar-isons.

Ventral nerve system

The ventral, paired or more or less fused nerveswith segmental ganglia develop in the metatro-chophores. They differentiate from 2d-cell descen-dants both in ‘‘polychaetes’’ and in clitellates(Shankland and Savage, ’97).

Protonephridia

A pair of protonephridia is found in bothplanktotrophic and lecithotrophic larvae. Eachconsists of a terminal cell, a duct cell, and a porecell, for example in Serpula and Harmothoe, butsome species have two or three terminal cells andsome have two to four duct cells. The terminalcells may be mono- or multiciliate, and some formshave terminal cells with many monociliateunits (Bartolomaeus, ’95, ’98). In Polygordius,Woltereck (‘04) observed that the early stages ofthe ‘‘Mediterranean larva’’ of P. neapolitanusand the ‘‘North Sea larva’’ (endolarva) ofP. lacteus or P. antennatus develop practicallyidentical protonephridia, and that these nephridiadevelop from the cells 3c222 and 3d222. This agreeswith the position of the nephridiopore in theperistomium of the other larvae which have beenstudied. The nature of the later, more complicated

larval nephridia of the ‘‘Mediterranean’’ and‘‘North Sea’’ larval types seems to be unknown.

Metamorphosis (Ectodermal structures)

Polychaete metamorphosis is usually a gradualprocess, because a few to many segments, whichmust be considered adult structures, develop inthe planktonic stage in most species. The larvalfeeding and locomotory ciliary bands are shed atthe end of the pelagic phase. The apical gangliondisappears, whereas the closely associated cerebralganglia, sometimes with eyes, remain as the adultbrain (see above).

The catastrophic metamorphosis in Owenia andPolygordius is discussed below.

Variation in larval types

The enormous variation of larval morphologywithin the polychaetes has already been men-tioned.

Planktotrophy is found in representatives of anumber of families, but both morphology andfunction of the feeding structures vary.

Ciliary filter-feeding with downstream-collect-ing bands of (usually) compound cilia (see Strath-mann et al., ’72, Riisgard et al., 2000) is found in anumber of species. It has been documented inlarvae of Serpulidae (Pomatoceros: Segrove, ’41;Serpula: Strathmann et al., ’72; Spirobranchus:Lacalli, ’84), Capitellidae (Mediomastus: Hansen,’93), Spionidae (Pygospio, Polydora: Anger et al.,’86), Opheliidae (younger larvae of Armandia:Hermans, ’78; Miner et al., ’99), Echiuridae(Urechis: Miner et al., ’99), and Oweniidae(Owenia: Emlet and Strathmann, ’94). The ros-traria larva of an amphinomid was studied byJagersten (’72), who observed proto- and meta-troch and particles transported along the adoralciliary zone to the mouth; this strongly indi-cates the presence of a downstream-collectingsystem. Larvae of the Polygordiidae (Polygordius:Woltereck, ’02) have the ciliary bands and arefeeding, but the particle capture has apparentlynot been studied in detail.

Young larvae of Armandia and Urechis captureparticles with the usual downstream mechanism,but older larvae, which have a much larger bodyrelative to the size of the ciliary bands, have beenobserved capturing larger particles with the ciliaaround the mouth (Miner et al., ’99). This seemsto be the only type of feeding in pectinariids, forexample, which have large, ciliated lobes at themouth (Rasmussen, ’73). Particle collecting with a


lateral ciliary ‘‘brush’’ (a specialization of a groupof prototroch cells left of the mouth) has beenobserved in polynoids (Phillips and Pernet, ’96).An unidentified spionid larva from the planktonnear Friday Harbor, WA (unpublished observa-tions by the author) has powerful compound ciliaon both sides of the mouth and seems perfectlyspecialized for capturing larger particles.

Larvae of Magelona are carnivores which catchbivalve and polychaete larvae with their long andflexible tentacles (Thorson, ’46); larvae of somespecies of Nephtys capture bivalve larvae by anunknown method.

The growth of the planktonic larvae of severalterebellids makes it clear that these larvae mustfeed in the plankton, but the mechanism(s) hasnot been studied. Chaetopterid larvae have beenobserved capturing particles on a pair of ciliatedlips and by a net of fine mucus strings secretedfrom the posterior part of the body (Werner, ’53).

Many polychaetes have lecithotrophic larvaewith a prototroch; several species also have atelotroch used exclusively for swimming. Directdevelopment is ecologically speaking present in afew species which have brood protection and inwhich there is no planktonic stage. Pygospioelegans shows a variation from the more normallecithotrophic larvae to large, hunchbacked em-bryos with direct development, apparently de-pending on the available amount of nurse eggs.However, both types develop prototroch andmetatroch and swim inside the egg capsules(Rasmussen, ’73). As mentioned above, somesabellid larvae have all the ciliary band necessaryfor ciliary filter feeding, and may capture particles,but mouth and gut are not fully developed andthe particles are ‘‘rejected’’ at the mouth areaand carried away along the gastrotroch (Pernet,2003).

Special names have been given to certain larvaltypes, especially to later ontogenetic stages (see forexample Heimler, ’88). In general, the termmetatrochophora is used for older larvae withsegmented body, and nectochaeta for pelagiclarvae in which the locomotion has been takenover by the parapodia, but these stages fall outsidethis review. However, one type of larva must bementioned, the pericalymma larva, because it hasbeen central in some phylogenetic theories. Apericalymma larva (or serosa larva) has most or allof the segmented adult body covered by a largeextension (serosa) from an anterior region. Twotypes have been reported from polychaetes(Nielsen, 2001): Type 2 with the serosa originating

from region just behind the metatroch (knownfrom Polygordius lacteus and P. appendiculatus(Wolterek, ’02) and some phyllodocids, probably ofthe genus Eulalia (Tzetlin, ’98; unpublishedobservations); and Type 3 with the serosa devel-oping from the posterior side of the first setiger(known from Owenia and Myriochele; see Wilson,’32; Emlet and Strathmann, ’94). Both types areplanktotrophic and have a ‘‘catastrophic’’ meta-morphosis, comprising shedding of larval struc-tures, such as proto- and metatroch. These two,quite different larval types occur in genera withinfamilies with more normal trochophores, so thereis no indication that a pericalymma type could beancestral within the annelids, as proposed, forexample, by Salvini-Plawen (’80).

Discussion of annelid development

The studies of cleavage patterns and cell-lineageof the ‘‘polychaetes’’ show very considerablesimilarities between the species. ‘‘Oligochaetes’’and hirudineans have direct development, andnone of them show any signs of ciliary bands orapical tuft. Their cleavage follows a modified spiralpattern with more or less strong bilateral tenden-cies in the later stages (Anderson, ’73; Dohle, ’99;Shankland and Savage, ’97).

Both cell-lineage studies and studies usingmarking of individual blastomeres indicate thatthe polychaetes must be interpreted as consistingof three larval regions, prostomium, peristomium,and pygidium, and a segmented body regiondeveloping from the growth zone at the anteriorside of the pygidium. The whole segmentedbody and the pygidium develop from cells of theD-quadrant, in particular the 2d-cell (Nielsen, inpress; Fig. 5).

In polychaetes the prototroch develops from thefour groups of primary trochoblasts (from all fourquadrants) plus three groups of secondary trocho-blasts (from the A-C quadrants); in some speciesalso from accessory trochoblasts. The cell-lineageof the metatroch has not been documentedconvincingly, but it appears to develop from 2d-cells in Polygordius. The telotroch probablydevelops from descendants of the 2d-cell (seeabove), i.e. from cells which have become ‘‘dis-placed’’ from the posterior part of the prototrochring. This is in complete agreement with thetrochaea theory (Nielsen, ’79, 2001) which en-visages the evolution of the three bands ofcompound cilia of the trochophore, proto-, meta-,and telotroch, through specialization of regions of

C. NIELSEN48

one circumblastoporal ring of compound cilia, viz.the archaeotroch in the hypothetical protostomianancestor called trochaea.

The structure of the ancestral annelid larva hasbeen discussed for more than a century, the firstserious attempt being that of Hatschek (1891).Many more or less well-supported ideas have beenput forward, but there now seems to be agreementthat the ancestral annelid had spiral cleavage anda prototroch, as found in most spiralians (seeRouse, ’99 for discussion). The main disagreementseems to be whether the ancestral larva wasfeeding or not. Mapping of larval types onto aphylogeny is problematic due to the lack of agenerally accepted phylogeny of the Annelida(compare Rouse and Fauchald, ’97; Westheideet al., ’99; Giribet et al., 2000; Peterson andEernisse, 2001; Struck et al., 2002; Bleidorn et al.,2003). However, the observations of two types ofciliary feeding in larvae of Armandia and Urechis(Miner et al., ’99; see above) with the downstream-collecting method used by the early stages indicatethat the various other types of ciliary feedingdescribed above are derived.

MOLLUSC DEVELOPMENT

Numerous authors have reported on the devel-opment of various molluscs and it seems clear thatall groups except the cephalopods show spiralcleavage (Fig. 7). The cephalopods have ratherlarge eggs with discoidal cleavage and direct

development (Boletzky, ’88); they will not bediscussed here.

Mollusc reproduction and development showenormous variations, ranging from free spawningof eggs and sperm, for example in most bivalves,to copulation and encasement of the fertilized eggsin elaborate, often species-characteristic cocoons,such as those found in many prosobranchs.

Some of the earliest attempts at establishing anotation for the cleavage pattern which we nowcall spiral were based on observations of thedevelopment of polyplacophorans and scaphopods(Kowalevsky, 1883a,b), and many later studieshave documented the presence of this pattern. Inhis famous study of the embryology of Crepidula,Conklin (1897) introduced the spiral cleavagenotation which soon became standard and re-viewed earlier studies of development of molluscs,annelids, turbellarians, and nemertines. He pro-posed that the cleavage patterns are of phyloge-netic value and pointed out that the prototrochdevelops from homologous cells in annelids andmolluscs. The literature on Solenogastres, Poly-placophora, and Gastropoda up to about 1960 wasreviewed by Hyman (’67). General reviews ofmollusc development and larvae are found inRaven (’66) and Fioroni (’82).

Early development and cell-lineage

Important reports on cleavage patterns and cell-lineage include studies on: Polyplacophora:Ischnochiton (now Stenoplax) (Heath, 1899),Acanthochiton (van den Biggelaar, ’96); Gastro-poda: Crepidula (Conklin, 1897), Trochus (Robert,’03), Physa (Wierzejski, ’05), Fulgur (now Busy-con) (Conklin, ’07), Littorina (Delsman, ’14),Lymnaea (Arnolds, ’82), Haliotis (van den Bigge-laar, ’93), Patella (Damen and Dictus, ’94; Dictusand Damen, ’97), and Ilyanassa (Render, ’97); seealso review in van den Biggelaar and Haszprunar(’96); Bivalvia: Unio (Lillie, 1895), Dreissena(Meisenheimer, ’01); and Scaphopoda: Dentalium(now Antalis) (van Dongen and Geilenkirchen,’74). (These references will not be repeated in thissection). No report exists on Caudofoveata andMonoplacophora. Robert (’03) reviewed earlierreports and gave useful translations of theirnotations.

Development is through lecithotrophic larvae ormore rarely direct (see discussion below) insolenogasters, caudofoveates, polyplacophorans,and scaphopods (and cephalopods), but bothgastropods and bivalves show much variation of

Fig. 7. Cell-lineage of Patella vulgata, based on Dictus andDamen (’97). Cells of the prototroch in boldface.


developmental types, from planktotrophic,through lecithotrophic to direct (see below). Thereis some correlation between egg size and develop-mental type, with small eggs generally giving riseto planktotrophic larvae and large eggs giving riseto lecithotrophic larvae (or direct development).However, cleavage seems to be total in all species.

Orientation of the embryo; Regulativepowers

The late oocytes in the ovary show a clearorientation, with a narrow basal part and aswollen apical part, together giving the ovary thelikeness of a bunch of grapes in some species(reviewed by Dohmen, ’92). The basal pole of theoocyte normally becomes the blastoporal (vegeta-tive) pole of the embryo and the apical polebecomes the apical (animal) pole where the polarbodies are given off. The orientation of the firstcleavage spindle can be influenced experimentally,and the orientation of the cleavages follows thenew orientation (Lillie, 1895; Guerrier, ’68, ’70).Oocyte development is arrested at some stage ofthe first meiotic division, so the polar bodies arenot given off until just after fertilization (Longo,’83). Lillie (1895, ’01) observed that the narrowattachment of the blastoporal pole to the ovarialwall in Unio leaves a small ‘‘micropyle’’ where theegg is attached to the vitelline membrane, andstated that this is the point of sperm entry. Thisappears indeed to be the case in Unio (Focarelliet al., ’88, ’90; Rosati et al., 2000), but in otherspecies, fertilization can occur at most areas of theegg (Raven, ’66; Longo, ’83).

Polyplacophoran eggs are surrounded by asometimes very elaborate structure called a hull(Eernisse and Reynolds, ’94) or chorion. Thisstructure is mainly secreted by the oocyte, butshaped by a surrounding layer of follicle cells. Itdoes not seem to form any micropyle, so the spermmay enter at any point of the egg.

Solenogasters and many gastropods have inter-nal fertilization, and the presence of follicle cellsaround the ripe egg may influence the point ofsperm entry into the egg, but very little is knownabout this.

The establishment of the secondary, anterior-posterior axis, and thereby, of bilaterality, hasbeen the subject of much discussion. It is clearlydetermined by the point of sperm entry in thebivalves Cumingia, Pholas, and Dreissena (Guer-rier, ’70; Morgan and Tyler, ’30; Luetjens andDorresteijn, ’98). The plane of the first cleavage is

through the primary axis and the spindle prefer-entially orients itself perpendicular to the spermentry point.

Many species show unequal cleavage with asmaller AB-cell and a larger CD-cell, and the fourcell stage often consists of three smaller, A-C cellsand a larger D-cell. The median plane of theembryo is through the B- and D-cells in suchembryos, and the orientation of the embryo hasclearly been established before the first cleavage.The primary axis plus the sperm entry pointcannot alone be responsible for the unequalcleavage; additional information must be presentin the egg, but this is poorly understood.

Species with equal cleavage are more difficult tointerpret. The orientation could have been estab-lished before the first cleavage, as in the unequallycleaving species, but without being expressedmorphologically in a manner that has beenobserved, or the orientation could become estab-lished at a later point in time (see also Freemanand Lundelius, ’92). A biradial pattern is seen atthe four-cell stage, with a long contact zonebetween the A and C cells at the apical pole anda long contact zone between the B and D cells atthe blastoporal pole (Arnolds et al., ’83).

Early cleavage stages show restricted powers ofregulation. Isolated AB-blastomeres of Dentaliumdevelop into embryos without post-trochal struc-tures, whereas the CD-blastomeres become morenormal embryos, but with abnormal proportions(Reverberi, ’71). Isolated blastomeres of the two-cell stages of a number of pulmonates regulatecompletely and develop into small but normallarvae (Verdonk, ’79).

Isolated micromeres of the first quartet ofDentalium (Wilson, ’04a) follow their normaldevelopmental pattern, each differentiating intoepithelial cells and four primary trochoblasts.Deletion experiments on eight-cell stages ofIlyanassa (Clement, ’67; Sweet, ’98) showed that,for example, eyes and tentacles, which normallydevelop from 1a and 1c, were absent in larvaewhere these blastomeres had been removed.Removal of the 2d-cell resulted in larvae withouta shell, and removal of 3c and 3d resulted in larvaelacking the right statocyst and right half of thefoot, and left statocyst and left half of the foot,respectively (Clement, ’86a,b).

Deletion or destruction of blastomeres at four-and eight-cell stages of pulmonates are moredifficult to interpret, but a few embryos developnormally after destruction of one blastomere(Morrill et al., ’73). A determination of the four

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quadrants has been demonstrated at the eight-cellstage of Lymnaea, when deletion of one of the firstquartet of micromere results in different blasto-mere configurations. Fairly normal veligers devel-op from embryos where a lateral, i.e. 1a or 1c,micromere has been deleted, and rather normalsnails develop from some types. Embryos with adeleted median micromere similarly developrather normally, and most veligers appear as if itis the 1b-micromere which has been removed. Thisindicates that the orientation of the embryo is notfixed at this stage (Arnolds et al., ’83). The finaldetermination of the anteroposterior (dorsoven-tral) orientation seems in some way linked to theestablishment of contact between one of themacromeres (which then becomes the 3D-cell)and some of the apical micromeres (van denBiggelaar, ’77, ’96). This can be influenced experi-mentally, but the underlying mechanisms remainunexplained.

Some regulatory powers are obviously presentin the earliest developmental stages, and thereseem to be considerable differences betweenspecies.

Experiments with centrifugation and compres-sion of eggs have shown that the orientation ofthe spindle can be modified and that the plane ofthe first cleavage follows the new orientation; thefollowing development proceeds normally (Guer-rier, ’80; van den Biggelaar and Guerrier, ’83;Goldstein and Freeman, ’97), but this does notexplain the origin of the axes in the undisturbedegg. Removal of the polar lobe disrupts theMAPkinase signaling which induces the organiza-tion of the embryo by the 3D-cell (Lambert andNagy, 2001), but again, this is a late event indevelopment.

First two cleavages

The cleavage shows the characteristic spiralpattern (Fig. 7) described for annelids (see above).It is holoblastic even in some species with quitelarge eggs, such as Busycon carica with eggs of adiameter of about 1.7mm.

The first two cleavages may be equal or unequal,and polar lobes of various sizes are formed inseveral species, with variation ranging from equalcleavage without polar lobes in Haliotis to highlyunequal cleavages where the polar lobe is of thesame size as the blastomeres, giving rise to the so-called trefoil stage in Ilyanassa (review in Verdonkand van den Biggelaar, ’83). Species with polarlobes must be considered unequal cleavers,

although the size differences are in some casesinsignificant. The unequal pattern becomes estab-lished in different ways in species with andwithout polar lobes. In species with a polar lobe,the spindle is situated in the middle of the zygoteand the two asters are of similar size, so the polarplasm must determine which cell becomes thelarger CD-cell. In some species with unequalcleavage without polar lobes, the spindle orientsperpendicular to the point of sperm entry butbecomes displaced to one side; this becomes thesmaller blastomere with a smaller aster (Lillie,’01; Guerrier, ’70). Polar lobes are also formed atthe third to fifth cleavages in a few species(Dentalium, Verdonk and van den Biggelaar, ’83;Littorina, Moor, ’73). There seems to be no generalsystematic pattern in the occurrence of polar lobes(Verdonk and van den Biggelaar, ’83).

First micromere quartet (1a–1d)

Ischnochiton shows an almost schematic spiralcleavage. The 1a111–1d111 cells become the apicalrosette, the two descendants of each of the cells1a122–1d122 become accessory trochoblasts, andthe four descendants of the cells 1a2–1d2 becomeprimary trochoblasts. No cells move behind theprototroch.

The above-mentioned gastropods show verysimilar cleavage patterns. A varying number ofcells at the apical pole develop cilia, which form anapical tuft in several species. The cells 1a2 –1d2 aredescribed as the primary trochoblasts, but thedevelopment of cilia has usually not been ob-served. Accessory trochoblasts are usually notmentioned, but they may have been overlooked.Four species of pulmonates, which all have directdevelopment and therefore a non-functioningprototroch, show fewer divisions of the prototrochcells (Damen and Dictus, ’94). Patella was followeduntil the trochophora stage with a fully differ-entiated prototroch, and the pattern turned out tobe more complicated (Damen and Dictus, ’94).Initially, 26 cells develop prototroch cilia: the twodescendants of each of the cells 1a122–1c122, thefour descendants of the cell 1d12 (accessorytrochoblasts), and the four descendants of eachof the cells 1a2–1d2 (primary trochoblasts); how-ever, some of the cells, mostly accessory trocho-blasts, subsequently deciliate. The further fate ofthis quartet has been studied in some detail inCrepidula. The cells 1a111–1d111 develop into asmall apical organ with short cilia. Lateral to thisorgan two groups of cells proliferate rapidly,


(A) (B)

(C) (D)

Fig. 8. Developmental stages of Ischnochiton magdalenen-sis (now Stenoplax heathi), based on Heath (1899). Colors ofthe quadrants: A: blue, B: yellow, C: green, D: red. The area ofthe future invagination is outlined in a heavy line. A-C,blastoporal views; D dorsal view. A, Stage of about 80 cells. Band C, Two stages of early gastrulation; the blastopore issharply defined anteriorly, except posteriorly where there is ashallow groove with D-cells at the bottom posteriorly. D, Earlytrochophore; the 2d-cells are red and the other D-quadrantcells pink.

ventral dorsal

Fig. 9. Early trochophore of Patella vulgata, based on Dictus and Damen (’97). Colors of the quadrants: A: blue, B: yellow, C:green, D: red (in D, descendants of the 2d-cell are red and the other D-quadrant cells pink). The narrow blastopore is indicatedby the black dot.

Fig. 10. Early trochophore of Dentalium in left view, basedon van Dongen and Geilenkirchen (’74). Colors of thequadrants: A: blue, B: yellow, D: red. The compound cilia ofthe prototroch are indicated as small ovals. The arrowindicates the position of the closed blastopore.

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extending below the epithelium forming theprimordia of the cerebral ganglia. The gangliaare connected via the basal part of the apical organthrough a slender commissure. A pair of eyesdevelop as small ectodermal invaginations next tothe ganglia. Trochoblasts were identified based oncomparison with other trochophore embryos, butcilia do not develop until a later stage when ahorseshoe-shaped ridge consisting of a severalhundred cells foreshadow the shape of the bilobedvelum. The cleavage pattern of Lymnaea wasstudied in detail by Arnolds (’82), and thedevelopment of eyes, tentacles, and cerebral gang-lia from the cephalic plates, i.e. descendants of thecells 1a1211, 1c1211, 1a1212, 1c1212, 1c1112, 1d1112,and 1b1212, was stated by Verdonk and van denBiggelaar, ’83). The development up to the feedingstage was studied in Ilyanassa by Clement (’67)and Render (’97), but individual blastomeres couldnot be followed. Using microinjection of blasto-meres, Render (’97) could show that the apicalplate, including eyes, and part of the velumdeveloped from cells of the first micromerequartet; cells of proto- and metatroch were notdistinguished, but the labeling of the first andsecond micromere quartets are consistent with theidea that the prototroch develop from cells of thefirst quartet and the metatroch from cells of thesecond quartet.

Bivalves show similar cleavage patterns, but thestudies are not very detailed.

In Dentalium, the 1a111–1d111 and 1a1121–1b1121

become the apical organ, with the apical tuftdeveloping from 1c1111 and 1d1111; 1a122–1c122 and1d12 become accessory trochoblasts, and 1a2–1d2

primary trochoblasts.

Second micromere quartet (2a–2d)

In Ischnochiton, the six descendants of 2a1–2c1

become secondary trochoblasts, although theciliation of the 2a12–2a12 cells was not described.The cells 2d1 and 2d21 form the somatic plate,which is quite narrow dorsally (Fig. 8). It formspart of the stomodaeum, part of the mantle withthe shells (or possibly the whole shell area asindicated in Heath’s fig. E), part of the foot, andpossibly the pedal nerve cords. It appears thatboth the proctodaeum and the surrounding telo-troch develop from these cells.

In Crepidula, the cells of the second and thirdquartet give rise to most of the posttrochalectoderm, with the 2d-cell probably giving rise tomantle, shell gland, and the main parts of the foot,

although this was not documented in detail. InPatella (Damen and Dictus, ‘94), the two descen-dants of each of the cells 2a11–2c11 (secondarytrochoblasts) initially develop cilia, but the cells2a112 and 2c112 subsequently deciliate. The smallciliary tuft near the anus, usually called telotroch,also derives from the 2d-cell. The remainingdescendants of the second micromere quartetbecome ectoderm, including the dorsal cell platewith the shell gland and the ventral cell plate withthe foot (see below), ectomesoderm, and stomo-daeum. Render’s (‘97) studies of Ilyanassa indi-cate that the posterior part of the velum with themetatroch develops from the 2a–2d cells inplanktotrophic veligers. Other descendants ofthese cells give rise to stomodaeum (from 2a–2c),mantle and foot, and ectomesoderm. Blastomeremarking in Patella embryos (Dictus and Damen,’97) showed some unexpected results for the originof the mantle and shell gland which are in contrastto the results summarized above (Fig. 9). Theyfound that only the midventral part of the embryodevelops from the 2d-cell, whereas the major, left,right, and dorsal parts of the body including theshell gland originate from the cells 2a, 2b, and 2c.The 2b-cell gives rise to a narrow ring around theposterior side of the prototroch and a small part ofthe shell gland. Unfortunately, the movementsof the blastomeres leading to the configurationobserved in the trochophore stage have not beenfollowed. However, a more detailed study of thelineage of the 2d-cell and other cells around theblastopore (Lartillot et al., 2002) has shown thatthe ventral midline of the late gastrula to earlytrochophore stages is occupied by a row of cellsgiven off from the 2d2-cell in a teloblast-likemanner, indicating the very same pattern ofblastopore closure as that observed in polychaetes.

Few authors have studied bivalves. In Dreissenaand Unio, descendants of the cell 2d11 form theshell gland; it soon forms a deep, transverselyelongate invagination, which is so deep it hassometimes been mistaken for the archenteron.

In Dentalium, the cells 2a1, 2b1, 2b21, 2c11, and2c21 become secondary trochoblasts, 2a212 and 2c122

become ectomesoderm, and the 2d-cells become thesomatic plate, including the mantle with the shelland the right foot anlage. The development wasonly followed until the early trochophore stage.

Third micromere quartet (3a–3d)

These cells are quite large in Ischnochiton andform stomatoblasts. Descendants of 3c and 3d


additionally form quite large lateral and latero-dorsal areas in the young larva (Fig. 8), but are notbelieved to become part of the area that forms theshells. They were reported to form a posteriorgrowth zone of small cells, but this should bestudied more closely.

In Ilyanassa, the 3a–3d cells give rise to part ofthe velum, foot, heart, and oesophagus (Render,’97). In Crepidula, the descendants of 3c and 3dform quite extensive areas lateral to the middorsalstripe of 2d-cells, but their participation in theformation of shell gland and foot was not docu-mented. In Patella, the cells derived from 3a and3b, unexpectedly, only give rise to ectomesodermalcells. The cells 3c and 3d give rise to narrow,symmetrical ectodermal areas lateral to the mouthand two small areas on the knob carrying the‘‘telotroch’’ (Fig. 9).

The studies on bivalves are not very detailed.In Dentalium, the descendants of 3a–3c become

ectoderm, 3d1 becomes incorporated into thesomatic plate, and 3d2 in the left foot anlage.

Fourth micromere quartet (4a–4d)

In all the molluscs studied so far, the cells 4a–4cgive rise to endoderm. In Ischnochiton andDentalium, the 4d-cell is said to become themesoblast, but this cell gives rise to both meso-derm and endoderm in Crepidula, Patella, andother gastropods (see also Verdonk and van denBiggelaar, ’83), and it cannot be excluded that asimilar fate has been overlooked in the otherspecies.

Macromeres (4A–4D)

The macromeres give rise to endoderm in allspecies.

Gastrulation, blastopore closure, andfurther development of the ectoderm

Gastrulation shows much variation, from in-vagination in species with yolk-poor eggs toepiboly in species with much yolk (Raven, ’66).The macromeres invaginate first, followed by the4a–4c-cells; the 4d-cell moves into the cleavagecavity at about the same time. The blastopore issurrounded by cells from all four quadrants, withalternating groups of cells of the second and thirdquartet; descendants of the 2d-cell occupy theposterior/dorsal side of the embryo which becomesgreatly stretched after gastrulation. The blasto-pore usually becomes quite narrow and well-defined, except posteriorly, where a groove with

D-quadrant cells at the bottom is formed in somespecies (Fig. 8). The blastopore may persist,becoming internalized by the differentiation of astomodaeum from cells from all four quadrants;other species show a complete closure of theblastopore and later development of the stomo-daeum. In most species, the blastopore/mouthmoves anteriorly, finally becoming situated justbehind the prototroch (in front of the metatrochwhen present). A proctodaeum develops later; itscell-lineage has not been well studied, but fromthe distribution of the cells of the D-quadrant itappears that it arises from 2d-cells. In Viviparus(¼Paludina), the blastopore apparently remainsin a posterior position and becomes the anus,while an independent stomodaeum breaksthrough at an anterior point; only the early cell-lineage was studied (Dautert, ’29; Fernando, ’31).

The type of blastopore closure has been dis-cussed at great length. Is the most common typethe ‘‘typical protostomian type of blastopore fate,’’i.e. the blastopore becomes the mouth (Grobben,’08; Hyman, ’51; Brusca and Brusca, 2003), andthe Viviparus example just an aberration, or is theposterior groove from the blastopore a reminis-cence of the ancestral state where the lateralblastopore lips fused and divided the blastoporeinto mouth and anus (Jagersten, ’55; Nielsen, ’85,2001)? Here, it must suffice to point out that themidventral area between mouth and anus hasbeen shown to be derived from the D-quadrant,probably from the 2d-cell, in all species wherecell-lineage has been studied; this is in goodaccordance with the blastopore-fusion theory (seefurther below).

Ciliary bands

Molluscs have ciliary bands composed of multi-ciliate cells. Compound cilia are found in manylarval types, which may have proto-, meta-, ortelotrochs (and in some types accessory ciliarybands). These ciliary bands generally have thesame structure and function as those of polychaetelarvae and are named accordingly, although thehomology of metatrochs has been questioned(Salvini-Plawen, ’80; Haszprunar et al., ’95). Thecompound prototroch cilia develop from separatecilia, which move close together and grow longer(Bellolio et al., ’93; Morse and Zardus, ’97).

Almost all planktonic mollusc larvae swim bymeans of the prototroch, and a prototroch or aprominent ring of trochoblasts is found in mostspecies with direct development inside an egg

C. NIELSEN54

capsule. Planktotrophy with a downstream-col-lecting system of prototroch, adoral ciliary zoneand metatroch is found in some gastropod andbivalves.

Polyplacophorans and some gastropods havea smooth muscle ring situated just basal tothe prototroch cells, but this muscle has notbeen found in other gastropods or in bivalvelarvae (Wanninger et al., ’99; Wanninger andHaszprunar, 2002). The ring nerve along theprototroch is discussed below.

Prototroch

Almost all non-cephalopod molluscs have aprominent, preoral band of compound cilia, theprototroch, and the ciliated cells have similarcell-lineages in all species studied. There seemsalways to be a number of primary trochoblasts,typically 16, whereas the number of secondary andaccessory trochoblasts varies between species. Thearrangement of the cells also varies.

The caudofoveate Chaetoderma (Nielsen, 2001)and the solenogasters Epimenia (Okusu, 2002)and Neomenia (Thompson, ’60) show a prototrochwith one ring of thick compound cilia, althoughNeomenia has pericalymma larvae (see below)where the prototroch zone is extended in a serosawith very large cells. In the polyplacophoranIschnochiton, Heath (1899) found eight accessory,16 primary, and six secondary trochoblasts. Thecompound cilia are usually arranged in two rows.

Most gastropods have the prototroch situated atthe edge of a velum, with the compound ciliaarranged in one or two rows (Werner, ’55;Chaparro et al., ’99). The cell-lineage has beenstudied in fine detail in Patella (Damen andDictus, ’94). Initially, 32 ciliated prototroch cellsdevelop, but a number of the cells subsequentlydeciliate, so that the fully formed prototrochconsists of one ring of 21 cells, including membersof all the three groups (see above). Trochus(Robert, ’03) has only 25 cells in the prototroch,viz. the 16 primary trochoblasts and the secondarytrochoblasts 2a111–2d111, 2a112–2c112, and 2a12111–2c12111. These cells initially form two horseshoe-shaped rows, with dorsal gaps, but they soonrearrange into one complete circle. Four species ofpulmonates, which all have direct development,show fewer divisions of the prototroch cells, andthe cell 2d11 becomes a secondary prototroch cell(Damen and Dictus, ’94).

Conklin (1897) tried to establish the cell-lineageof the prototroch in Crepidula. The cells 1a2– 1d2

were shown to take part in the formation ofthe prototroch together with cells from thesecond quartet, but the number of cells is highand the cells of the second quartet could not befollowed. There is a wide dorsal break in theprototroch (and metatroch), which indicates thatthe cells of the 2d-field do not participate (as inpolychaetes). The detailed studies on Patella havebeen mentioned above. There is one row ofcompound cilia in the prototroch of Patella andCrepidula (Werner, ’55).

The cell-lineage of the bivalve prototroch hasnot been documented. Two rows of compound ciliaare seen in full-grown veligers of Pecten andOstrea (Waller ’81; Cragg ’89). The full-grownlarva of Dentalium shows a prototroch with 40cells: primary trochoblasts and accessory trocho-blasts from all quadrants, and secondary trocho-blasts from A-C, forming three rings of compoundcilia (van Dongen and Geilenkirchen ’74; Wann-ninger and Haszprunar, 2001; see Fig. 10).

The presence of one, two, or three rows ofprototroch cilia is related to the original develop-ment of the three types of trochoblasts, which mayremain in their original position, as the three rowsin Dentalium, or become rearranged into one row,as in Patella.

It is generally believed that the trochoblasts donot divide, but veligers of many gastropods andsome bivalves (for example Atrina and Plankto-mya) have very extended velar edges with folds orlobes. The number of prototroch cells mustdefinitely be higher than that observed in theearly stages, but it is not known how the highernumber is achieved.

Adoral ciliary zone

A band of multiciliate cells with rather short,separate cilia is found between proto- and meta-troch in all planktotrophic larvae. Homologousciliated areas may be found behind the prototrochin non-planktotrophic species, but their identityhas not been ascertained.

Metatroch

A well-developed metatroch is found in allplanktotrophic gastropod and bivalve larvae. Itconsists of compound cilia in all species whichhave been investigated more closely, see forexample Nielsen (’87), Buckland-Nicks et al.(2002), and Zardus and Martel (2002).


Gastrotroch

The ciliated foot found in most molluscs devel-ops along the ventral midline behind the mouth,and the cell-lineage studies of Patella have shownthat it develops from the 2d-cell (Dictus andDamen, ’97). This indicates homology with thegastrotroch in annelid larvae, but the ciliationdoes not function as a rejection band in any of themollusc larvae.

Telotroch

The larvae of the few solenogasters (Epimenia,Neomenia, Okusu, ’02; Thompson, ’60) and thecaudofoveate (Chaetoderma, Nielsen, 2001) thathave been studied all show a telotroch of com-pound cilia around the anal region. Some gastro-pod trochophores have a small tuft of cilia justbehind the closed blastopore, and this is usuallyinterpreted as a telotroch (Tomlinson, ’87). InPatella, this structure develops from 2d-cells(Dictus and Damen, ’97) as does the telotroch ofannelid trochophores.

Additional ciliary rings

A pretroch consisting of short, separate ciliasituated just apical to the prototroch has beenobserved, for example in polyplacophorans(Nielsen, ’87), gastropods (Werner, ’55), andbivalves (Cragg, ’89, ’96). Its cell-lineage has notbeen studied.

Beautiful examples of additional rings ofcompound cilia around the body behind themetatroch are seen in larvae of several pteropods,such as Pneumodermopsis (Nielsen, ’87) andClione (Buckland-Nicks et al., 2002).

Nervous system

The nervous system of molluscan larvae com-prises an apical/dorsal complex, with an apicalganglion and paired cerebral ganglia, and ventralnerves, which are continuous cords in mostaplacophorans, polyplacophorans, and monoplaco-phorans, whereas the perikarya are concentratedin ganglia in most of the conchiferans.

Apical organ and adult brain

The apical organ appears to develop from thecentral cells of the episphere, viz. 1a111 to 1d111,although the lineage has only been followed in afew species (Crepidula, Conklin, 1897).

An apical tuft of cilia has been reported fromalmost all types of molluscan larvae. Eernisse (’88)

reported a variation among polyplacophoran lar-vae from planktonic larvae with a long apical tuftof several compound cilia to brooded larvae with ahardly discernible field of short cilia. Apical organsof varying types, ranging from two compound ciliato a group of compound cilia, to an apparentlyundifferentiated ciliated area, are known fromsolenogasters, gastropods, bivalves, and scapho-pods (Conklin, 1897; Cragg, ’96; Okusu, 2002;Wanninger and Haszprunar, 2001). In somespecies, the young larvae have a long apical tuftwhich is lost in older larvae (Cragg, ’96; Page,2002).

The function of the apical organ has always beenassumed to be sensory because the apical organ isthe first part of the larva to come into contact withnew environmental cues during normal swim-ming. Both chemical and mechanical senses havebeen assumed.

The early larvae of the polyplacophorans showfive, and later 7–8, serotonergic cells in the apicalorgan, connected with the (FMRFamide-positive)cerebral commissure (Friedrich et al., 2002;Voronezhskaya et al., 2002). No innervationof the prototroch by serotonergic nerves wasobserved.

Kempf et al. (’97), Page and Parries (2000), andSchaefer and Ruthensteiner (2001) describedapical organs of some gastropods in fine detail.They found four main types of cells, ciliary tuftcells (apparently non-sensory), ampullary sensoryneurons (with a group of cilia in a deep, narrowinvagination), para-ampullary sensory neurons(with two short cilia at the surface of theepithelium), and non-sensory neurons. The axonsof sensory and non-sensory neurons form amedian neuropil in close contact with the cerebralcommissure. In nudibranch larvae, three of thepara-ampullary sensory cells and two of the non-sensory neurons are serotonergic. The sensorycells are arranged in three clusters. Serotonergicaxons extend in a branching pattern from theapical complex to the edge of the velum and alongthe prototroch cells. Kempf et al. (’97) suggestedthat the para-ampullary cells are mechanosensoryand that their axons to the velum could modulatevelar muscle activity. Page (2002) found a similarapical organ in the patellogastropod Tectura, butwith only one ampullary cell, with a pair ofconspicuous ciliated papillae lateral to the apicaltuft, and a with ciliated pit just behind it. Lessdetailed observations on apical organs of bivalvelarvae indicate similarity (Cragg and Crisp, ’91;Tardy and Dongard, ’93). The occurrence of nerve

C. NIELSEN56

cells along the prototroch of veligers is welldocumented (Croll et al., ’97; Kempf et al., ’97).Hadfield et al. (2000) studied larvae of theopisthobranch Phestilla and found that whensome cells of the apical organ had been destroyed,the larvae were unable to detect the metabolitesfrom their coral host and to go through metamor-phosis.

Thus, it seems that the apical organ has bothmechanical and chemical sensory cells, but thefunction of the apical tuft is still unknown.

The origin of the adult brain with the pairedcerebral ganglia has been observed in only fewspecies. Okusu (2002) described how a pair ofciliated depressions on the episphere of Epimeniabecome internalized and form the cerebral gang-lia. Similarly, it has been observed that in lategastropod embryos two small lateral groups ofcells of the episphere near the apical organ dividerapidly and sink below the ectoderm to form thecerebral ganglia (Conklin, 1897; Delsman, ’14;Jacob, ’84; Page, ’92). An eye cup develops from asmall ectodermal invagination at the lateral side ofeach cerebral ganglion. In Lymnaea, cerebralganglia, eyes, and tentacles develop from thecephalic plates, i.e. descendants of the cells1a1211, 1c1211, 1a1212, 1c1212, 1c1112, 1d1112, and1b1212 (Arnolds, ’82; Verdonk and van den Bigge-laar, ’83). Median extensions from the two gangliafuse and form the cerebral commissure, whichsoon contacts an extension from the apicalorgan. Serotonergic cells were observed in earlycerebral ganglia and the cerebral commissure veryclose to the apical organ in nudibranch larvae(Kempf et al., ’97).

Lin and Leise (’96a) tabulated the developmentof ganglia in larvae to juveniles of the prosobranchIlyanassa and found that the apical gangliondevelops first, followed by cerebral and pedalganglia. They also described histochemical evi-dence for NADPH-diaphorase, an indicator ofnitric oxide synthase activity, within the neuropilof developing ganglia. Enzyme activity was parti-cularly high within the apical ganglion as thelarvae approached metamorphic competence, butactivity was lost as the apical organ disappeared atmetamorphosis (Lin and Leise, ’96b). Thavarad-hara and Leise (2001) have subsequently givenimmunocytochemical evidence for nitric oxidesynthase within the larval apical ganglion ofIlyanassa.

The serotonergic nervous system of scaphopodlarvae resembles that of gastropods describedabove, but deviates in that it degenerates before

metamorphosis (Wanninger and Haszprunar,2003).

Eyes

Various pigmented spots are traditionally inter-preted as eye spots. Many polyplacophoran larvaehave a pair of lateral eye spots situated just behindthe prototroch, consisting of one to four rhabdo-meric receptor cells and a number of pigmentcells, innervated from the lateral nerve cord(Rosen et al., ’79; Bartolomaeus, ’92b). These eyesare lost soon after metamorphosis. Many gastro-pod larvae have eye spots or eyes situated at thebase of the tentacles, in contact with the cerebralganglia, i.e., at the episphere (Buckland-Nickset al., 2002). The eyes are smaller or larger andhave ciliary or rhabdomeric receptor cells, and insome cases a small lens (see Blumer, ’96). InLacuna, the cup-shaped eye spots consist of twosensory cells, one ciliary and one rhabdomeric, apigment cup cell, and a lens cell (Bartolomaeus,’92b). The eyespots are apposed to the cerebralganglia, surrounded by a common basal mem-brane. The eyes are retained at metamorphosisand develop into the adult eyes. Some species haveciliary receptor cells in the planktonic larval stage,but change to or add cells of the rhabdomeric typeat metamorphosis (Blumer, ’96, ’99) Larvae ofseveral species of pteriomorph bivalves developpigmented eye spots behind the prototroch, in-nervated from the cerebral ganglia, and subse-quently situated near the base of the developinggills (Cragg, ’96). These eyes are cup-shaped andcomprise an array of microvillar sensory cells andpigment cells (Rosen et al., ’78).

The different types of receptor cells and thedifferent position and innervation of the eye spotsdo not indicate homology.

Ventral nerve system

The adult ventral nervous system comprisesnerve cords with a number of somewhat irregularganglia in some solenogasters, whereas otheraplacophorans, polyplacophorans, and monoplaco-phorans have largely unganglionated cords; con-chiferans generally have more well-definedganglia, and pleural, pedal, parietal, and visceralganglia can usually be recognized, although theymay fuse more or less completely (Hyman, ’67;Salvini-Plawen, ’72).

The ‘‘ventral’’ ganglia develop during the larvalperiod, but since they are primarily related to the


juvenile-adult phase, they will only be treatedsummarily here.

The apical organ is traditionally treated as thefirst larval sense organ, but the first sensory cellsto develop both in gastropods (Raineri, 2000; Crolland Voronezhskaya, ’96; Dickinson et al., ’99,2000) and bivalves (Raineri, ’95; Raineri andOspovat, ’94; Croll et al., ’97) are actually one ora pair of ‘‘pioneer’’ cells at the posterior pole of thelarva near the rudiment of the foot; these cells arerecognized by their reaction to antibodies againstacetylcholine and FMRFamide. The ciliated gang-lionic cells send axons towards the apical pole, andbecome connected to the developing cerebralganglia. In the Mytilus trochophore, Raineri (’95)observed that a series of ganglia became organizedalong the two axons, viz. pedal ganglia at theposterior pole, and visceral, parietal, and pleuralganglia further anteriorly. In later veliger stages,the foot with the pedal ganglion moves forwards sothat the fused viscero-parietal ganglia become themost posterior pair of ganglia. In Patella, a similarorganization of the early nervous system has beenfound (Raineri, 2000), but the chain is stronglycurved in connection with the very early anteriordisplacement of the foot. The ganglia werereported to develop from ciliated epithelial cells.Surprisingly, posterior pioneer cells have not beenobserved in polyplacophorans, where the nervecords appear to develop along axons originating inthe apical/cerebral area (Voronezhskaya et al.,2002). The pedal nerve cords with commissuresdevelop first and the lateral cord soon after. Nervecells arranged in ganglia have not been observed(Friedrich et al., 2002).

The early development of ventral ganglia andcommissures in opisthobranchs has been studiedby Kempf et al. (’97) who observed serotonergiccells in pedal ganglia concomitant with thedifferentiation of the cerebral ganglia.

The origin of the ventral nervous system fromcells near the posterior pole indicates that the cellsoriginate from 2d-cells, but this has not beenproven.

The ‘‘double’’ ventral nervous system of un-ganglionated nerve cords in most aplacophorans,polyplacophorans, and monoplacophorans intui-tively appear more ‘‘primitive’’ than the mostlyganglionated system of the ‘‘higher’’ testaceans,but there is a very considerable variation in themorphology of molluscan CNSs (see Hyman, ’67;Bullock and Horridge, ’65;) and the ontogeneticdata available at this point give no hint of theancestral condition or of its differentiation.

Mantle and shell(s)

The mantle is a specialized dorsal epitheliumwith a thickened cuticle with calcareousspicules in aplacophorans, spicules and eightshells in chitons, and one or two shells in mostconchiferans.

The mantle edge can be recognized in scanningelectron micrographs of aplacophoran larvae, andit can be seen that the mantle edges move close tothe ventral midline in the solenogaster Epimenia(Okusu, 2002) and fuse in the caudofoveateChaetoderma (personal observations).

In polyplacophoran larvae, the posterior part ofthe mantle (behind the prototroch) with theposterior seven shells become visible first, whereasthe anteriormost part with the anterior shell,situated across the prototroch, usually developslater (Pearse, ’79). The mantle clearly developsfrom cells of the D-quadrant in Ischnochiton(Heath, 1899), but the participation of cells ofthe C-quadrant remains uncertain.

In the conchiferans, the mantle is restrictedto the posttrochal region, and all reports agreeon the existence of an unpaired shell gland,which subsequently becomes divided in thebivalves (Casse et al., ’97). In the gastropodsCrepidula, Busycon, and Trochus (Conklin, 1897,’07; Robert, ’03), the mantle and shell gland issupposed to derive from 2d-cells, but directobservations are lacking. However in Patella(Dictus and Damen, ’97; van den Biggelaar et al.,2002), cell-marking studies have demonstratedthat mantle and shell gland develop mainlyfrom 2a- and 2c-cell descendants, with a smallercontribution from 2b (Fig. 9). Unfortunately,it is not known how the various cells have cometo occupy the positions seen in the Patellatrochophore, but it seems reasonable to supposethat the 2a- and 2c-areas have moved dorsallyand ‘‘cut’’ the area of 2d-cells in two, ananterior (pretrochal-trochal) part and a ventralpart.

In Dentalium, the shell gland apparently devel-ops from descendants of the cell 2d1121, viz.2d112111 and 2d112121 (van Dongen and Geilen-kirchen, ’74; see also Wanninger and Haszprunar,2001).

Both Dreissena (Meisenheimer, ’01) and Unio(Lillie, 1895) show the shell gland developingfrom the 2d-cell, in Unio especially from 2d111

already at the gastrula stage. The gland was solarge that it was sometimes confused with thearchenteron.

C. NIELSEN58

Protonephridia

Protonephridia have now been observed inlarvae of caudofoveates (Chaetoderma, Dr B.Ruthensteiner, Zoologische Staatssammlung,Munich, Germany, personal communication),polyplacophorans (Lepidochiton, Bartolomaeus,’89), gastropods (many species, see for example,Bartolomaeus, ’89; Page, ’94, ’66; Haszprunar andRuthensteiner, 2000), bivalves (Mytilus, Branden-burg, ’96), and scaphopods (Antalis, Ruthenstei-ner et al., 2001). There is some variation inthe morphology of the protonephridia, from anunusually simple type consisting of only twocells in Antalis, to the supposedly primitivetype with a terminal cell, a ciliated duct cell, anda pore cell found in several groups, to organswith 15–20 ciliated duct cells in polyplacophoransand some pulmonates (Ruthensteiner et al.,2001). It must be taken for granted that protone-phridia were present in the molluscan ancestor,and the observed variations are consistentwith the idea that the ancestral protonephridiumconsisted of three cells (Bartolomaeus andAx, ’92).

Metamorphosis (Ectodermal structures)

The few aplacophorans studied so far show aconsiderable variation in types of metamorphosis.Epimenia (and probably also Chaetoderma) show agradual change, with the prototroch being dis-carded at the end of the planktonic phase (Okusu,2002). Neomenia and Nematomenia, which havepericalymma larvae, show a more dramatic meta-morphosis, where the serosa is rolled back andbecomes internalized in the episphere (Pruvot,1890; Thompson, ‘60); the fate of the apical organand cerebral ganglia has not been followed.

Polyplacophorans have a very gradual meta-morphosis, where the prototroch cells becomepushed out and lost (Ischnochiton, Heath, 1899;Eernisse, ’88).

Most of the gastropods with indirect develop-ment show a rather abrupt metamorphosis, withloss of the larval locomotory structures, i.e. theprototroch. The species with planktotrophic lar-vae, in particular, go through a stage of majorreorganization, necessitated by the change notonly in habitat, i.e. from plankton to benthos, butalso in feeding biology, from planktotrophy toother types of feeding, such as macrophagy,detritus feeding, or parasitism.

An example of a very rapid metamorphosis hasbeen well documented in Polynices, where the

change from planktotrophy to carnivory involvesthe shedding of both velum and larval oesophagusand the activation of preformed adult structures,such as a buccal mass with jaws and radula (Pageand Pedersen, ’98). The shell-less ‘‘opistho-branchs’’ shed the larval shell at metamorphosis(Thorson, ’46).

Most larvae of autolamellibranch bivalves areplanktotrophic and shed their velum at themetamorphosis from a pelagic, planktotrophiclarva feeding with the cilia of the velum to benthicadults feeding with the cilia of the gills. Thetransitory stage may be short, but is prolonged, forexample in Mytilus which has a pediveliger stage,and is able to shift between swimming andcreeping movement (Bayne, ’65). Protobranchbivalves have lecithotrophic pericalymma larvae(see below) which go through a catastrophicmetamorphosis with shedding of the serosa.

The apical ganglion, which seems mainly to beresponsible for the coordination of velar move-ment and for chemical testing in relation tosettling, apparently degenerates at metamorpho-sis. The cerebral ganglia are retained as a maincomponent of the adult brain.

Variation in larval types

Almost all molluscs (except the cephalopods)have ciliated larvae, but only gastropods andbivalves have the veliger-type larva, which oftenhas been considered the ‘‘characteristic’’ mollusclarva. The velum is a wing-like expansion of theprototroch-metatroch region, although the velumis actually quite narrow in most bivalve andopisthobranch larvae. The scaphopod larvae havea slightly expanded prototroch area, and one coulddiscuss whether this should be called a velum,although it has only the prototroch. The ‘‘velum’’of the three groups are formed by homologousstructures (see above), but it is questionablewhether the velum of the three groups as suchare homologous. Solenogasters, caudofoveates,polyplacophorans, and scaphopods have lecitho-trophic larvae with a prototroch, and in somegroups also a telotroch.

Gastropods show a large variation in develop-mental types. Many ‘‘archaeogastropods’’ havenon-feeding planktonic larvae, but many ‘‘meso-gastropods’’ have planktotrophic larvae with thecharacteristic downstream-collecting ciliary sys-tem of prototroch, adoral ciliary zone, and meta-troch (Werner, ’55; Riisgard et al., 2000). Severalmarine, most freshwater, and almost all terrestrial


gastropods lay egg capsules in which the embryosgo through a stage with a ciliated or unciliatedvelum before they emerge as small juveniles; thisis ecologically described as direct development(Hickman, ’99), but the presence of a seeminglynon-functional larval velum strongly indicatesthat this is derived from the ancestral, indirecttype of development with a planktotrophic larva(Hadfield and Iaea, ’89). Phylogenetic studies ofmajor clades indicate that planktotrophy/indirectdevelopment has been lost in many lineages. Thisis clearly the case in the genus Conus (Duda andPalumbi, ‘99) and in the Littorinoidea, in whichthere is an additional example of species whichappear to have regained the indirect development,viz., Lacuna (Epheria) within the Lacunidae (Reid,’89). Lieberman et al. (’93) demonstrated not onlythat planktonic larvae is the ancestral trait in theTurritellidae and that non-planktonic develop-ment has evolved many times, but also the generaltrend that non-planktonic lineages only rarelyrevert to planktonic development.

Bivalves (see Giribet and Wheeler, 2002 forclassification) show two types of larvae, theprotobranchs have lecithotrophic pericalymmalarvae (see below), whereas the autolamelli-branchs in general have planktotrophic veligerlarvae. Planktotrophic bivalve larvae are generallysmall with a circular to oval prototroch not muchlarger than the shell, but a few species have largerlarvae, for example Atrina and Planktomya, inwhich the velum is expanded and lobed to extendthe length of the ciliary bands (Zardus and Martel,2002). The lobed shape of the velum of largegastropod and bivalve larvae may be a prerequisitefor attaining large sizes, because body volume, andthereby metabolism, in principle increases at arate of the cube of body diameter, whereas thelength of the ciliated band of a circular or ovalvelum increases only directly with diameter. Onlythrough an extension of the length of the ciliarybands, either at the edge of a very large almostcircular velum, as in Planktomya, or at lobes of thevelum, as in larvae of prosobranchs and Atrina,can particle collection keep pace with increasingbody size.

The larval type called pericalymma Type 1 (seeNielsen, 2001) is found in some solenogasters andbivalves and should be discussed here, because ithas been central in some phylogenetic discussions(see for example Salvini-Plawen, ’80).

The solenogaster Epimenia (Okusu, 2002) has arather wide prototroch zone and the developmentof the hyposphere is somewhat retarded, but the

larva must be characterized as a rather normal,lecithotrophic trochophore with normal proto-troch and telotroch. In Neomenia (Thompson,’60), the development of the hyposphere isretarded and takes place in a posterior invagina-tion (pseudoblastopore). A ring-shaped infoldingof the hyposphere epithelium extends anteriorly,separating the outer serosa, with one ring oflarge prototroch cilia, from the hyposphere,which elongates and differentiates, finally witha telotroch surrounding the proctodaeum. Thestomodaeum develops from the innermost, ante-roventral part of the invagination. At metamor-phosis, the serosa reflects, exposing thehyposphere, and becomes internalized togetherwith the whole pretrochal region, although thefate of the apical organ has not been documented.A similar larval type is seen in Nematomenia(Pruvot, 1890).

The protobranch bivalves studied so far all haveType 1 pericalymma larvae (Yoldia and Nucula,Drew, 1899a,b; Solemya, Gustafson and Reid, ’86;Gustafson and Lutz, ’92; Acila, Zardus and Morse,’98). A serosa consisting of rings of thick, ciliatedcells cover the outside of the full-grown larva, theciliation consisting of scattered compound cilia inSolemya whereas Yoldia and Acila show threerings of cells each with a ring of compound cilia.The cell-lineage has not been followed, but thethree rings resemble those of Dentalium, whichare prototroch cells (see above, and Fig. 10). Thedevelopment of the permanent ectoderm secretingthe valves underneath the ciliated test cells hasnot been documented, and Drew’s (1899a,b) reportof a long tubular stomodaeum should be checked.

Okusu (2002) discussed the significance ofpericalymma larvae found in a number of phylaand concluded that the test-cell larvae couldhave evolved independently in each clade,and this is in full agreement with my earlierconclusions (Nielsen, ’95, ’98, 2001).

Discussion of mollusc development

Cleavage and cell-lineage show quite similarpatterns in all molluscs studied so far (except thecephalopods). The prototroch is formed from cellsof identical groups in all the classes, and althoughthe study of Patella (Dictus and Damen, ’97)shows a deviation from the pattern observed in thelineage of cells forming mantle and shell in othermolluscs, it seems clear that the body behind theperistomial area, i.e. behind the metatroch whenpresent, develops from cells of both C and D

C. NIELSEN60

quadrants. The cell-lineage of the metatroch of theplanktotrophic larvae has unfortunately not beendocumented. The somatic plate, i.e. the cellsoriginating from the 2d-cell, usually forms themid-dorsal surface of the developing embryoincluding the shell gland when present, and itextends to the posterior part of the blastopore sothat other important parts of the mid-ventralsurface, including the foot, the proctodaeum, andpossibly the ventral nerves, is formed from thisgroup of cells. The repeated units observed inshells of polyplacophorans, spicules of a solenoga-ster larva (Scheltema and Ivanov, 2002), andmuscles of polyplacophorans and monoplacophor-ans (Haszprunar and Schafer, ’97; Wanninger andHaszprunar, 2002) are not the result of addition ofsegments from a posterior growth zone and seemtherefore not to be homologous to the segmenta-tion of the annelids.

The apical ganglion comprises a few serotoner-gic cells in most species investigated, but it seemsdifficult to make generalizations about its func-tions. It has been supposed that the serotonergiccells control the muscle activity of the velum ingastropod larvae, but the function in other classesis unknown; they do not reach the prototroch inpolyplacophorans. The apical ganglion disappearsat metamorphosis (or a little earlier). The cerebralganglia develop from a pair of ectodermal thicken-ings lateral to the apical ganglion and are retainedin the adult brain. The function of the ciliary tuftis still unknown, and more studies of bothstructure and function of the apical organ inrepresentatives of several classes would be mostwelcome.

Cladistic analyses of the occurrence of plankto-trophy among molluscs seem inevitably to lead tothe result that planktotrophy has evolved severaltimes within the phylum, viz. in the autolamelli-branchs and a number of times in the gastropods(Hickman, ’99). However, this is based on theassumption that the gain of a complicated struc-ture such as the downstream-collecting ciliarycomplex is just as easy as the loss; this appearsquite unreasonable (Wray, ’95). Analyses ofsmaller clades, such as littorinioid, turritellid,and conid gastropods (Reid, ’89; Duda and Palum-bi, ’99; Lieberman et al., ’93) have shown that non-planktotrophy has evolved many times withingroups which apparently have planktotrophy asthe ancestral character, and it would perhaps bewise to use the information from such studies onthe phylum level. Chaffee and Lindberg (’86)studied Early Cambrian molluscs and concluded

that their small shell-size indicates a smallnumber of eggs and therefore direct development.However, as pointed out earlier (Nielsen, ’95), thepredation pressure in the plankton, which shouldbe the driving force behind this correlation, wasdefinitely not the same as in modern seas.Furthermore, there are recent examples of tinygastropods with planktotrophic larvae, for exam-ple Caecum glabrum (Gotze, ’38), which has a soft-part volume of only about 0.025mm3, so the small,early gastropods could well have had plankto-trophic larvae.

GENERAL DISCUSSION OF ANNELID ANDMOLLUSC DEVELOPMENT

Annelid and mollusc larvae are the mostintensively studied spiralian larvae, and it is easyto find many structures which are homologous.However, many of the homologies are shared withother spiralians, and it seems likely that more willbe identified by future studies. Therefore, I willhere focus more on the differences than on thesimilarities.

Cleavage patterns and most of the cell-lineagesshow detailed similarities, but it seems importantto point out that the whole annelid body behindprostomium and peristomium, i.e. all the seg-ments formed from the budding zone, is coveredby ectoderm derived from the 2d-cells. In themolluscs, the mid-ventral area is derived from the2d-cells, but the lateral parts of the body originatefrom other cells, i.a., the 3c and 3d-cells. Thisspeaks against the idea of an ancestral metameryof the molluscs, homologous to that of theannelids. This is in good agreement with theconclusions from studies of development anddifferentiation of nervous system and mesodermin polyplacophorans, the molluscan groupwhich shows the most conspicuous signs of‘‘segmentation’’ (Wanninger and Haszprunar,2002; Friedrich et al., 2002).

It should be emphasized that my previous,somewhat imprecise, statements about the persis-tence of the ‘‘apical organ’’ (apical ganglion pluscerebral ganglia) through metamorphosis in pro-tostomes/spiralians (Nielsen, ’95, ’98, 2001) mustbe modified. The apical ganglion is a larvalstructure which degenerates at metamorphosis,whereas it is the closely associated pair of cerebralganglia which develop in the larva and areretained through metamorphosis to become theadult brain. Both the apical ganglion and the


cerebral ganglia have almost identical origins inthe two phyla.

The continuing discussion about whether plank-totrophy or non-planktotrophy is the ancestralcharacter state of annelids and molluscs, andindeed of the protostomes in general, cannot beresolved through studies of individual phyla. Asmentioned above, the prototroch of the twophyla are unquestionably homologous, but theputative ancestral character of the metatroch canonly be discussed in the light of more general ideasabout evolution and ecology. However, thereseems not to be any character that excludes thepossibility of a planktotrophic larva in the lifecycle of the ancestral annelids and molluscs (seealso Wray, ’95; Nielsen, ’98, 2001; Strathmannand Eernisse, ’94).

ACKNOWLEDGMENTS

I am indebted to Dr. Louise Page (Victoria,Canada), Dr. Jo van den Biggelaar (Utrecht, TheNetherlands), and Dr. Ronald Jenner (Cambridge,UK) for comments on drafts of the manuscript andto Mrs. Birgitte Rubæk (Copenhagen, Denmark)for fine help with the illustrations.

APPENDIX: HOW I BECAME INTERESTEDIN TROCHOPHORA LARVAE AND

ANIMAL PHYLOGENY

The project for my degree from University ofCopenhagen (Marine Biological Laboratory, Hel-singør) concerned systematics and biology ofDanish entoprocts. Before I started my studies,four species had been reported from Danishwaters (one turned out to be a misunderstand-ing). There are now 26 known species (Nielsen,’64, ’89). The majority of the species aresolitary, belonging to the genera Loxosoma andLoxosomella; almost all of these species arecommensalistic and highly host specific.

Just after receiving my degree, I spent fivemonths at the Institute of Marine Science (nowRosenstiel School of Marine and AtmosphericScience), University of Miami, Florida, on ascholarship from the Nordic Council for MarineBiology. My plan was to study how loxosomatidlarvae select their host organism.

I soon found an (undescribed) species of Loxo-somella which was common on the surface of asponge at the nearby marina and which seemedideal for the experiments. However, it turned outto be more complicated than expected to grow the

small sponges needed for the experiments, andmore important, the larvae did not go through ametamorphosis resembling that described in allthe textbooks. The well-known metamorphosis ofPedicellina involves the settling of the larva and apeculiar rotation of the gut, which becomes the gutof the first zooid in a new colony. But the larva ofthe new species, subsequently described under thename Loxosomella vivipara, did not metamor-phose, but burst open to release a small juvenile,apparently formed through some hitherto un-known ‘‘internal’’ budding process, and then died(Nielsen, ’66).

This new observation fascinated me much morethan the experiments with host choice, so I made ahistological study of the budding process, whichturned out to bear considerable resemblance toearly stages of the budding in ectoproct bryozoans.This lead to extensive phylogenetic speculations,because for the past century, the two groupsEntoprocta and Ectoprocta had been consideredonly very remotely related. However, I decided topropose that the two phyla could be sister groups(Nielsen, ’71).

The idea that entoprocts and ectoprocts couldbe sister groups immediately leads to the questionabout their relationships to other phyla, andseveral years’ SEM studies on the ciliary bandsof a variety of invertebrate larvae (Nielsen, ’79,’87) gave me the idea that protostomes anddeuterostomes each have a characteristic larvaltype, viz. trochophora and dipleurula, respectively.This in turn led to a discussion of the evolution ofthe animal phyla (Nielsen, ’85).

During this period I increasingly felt the needfor a text which would summarize importantphylogenetic information, both on adult morphol-ogy and ontogeny, and discuss various possibilitiesfor reconstructing animal radiation. After aboutfive years these studies and speculations werepublished as the book ‘‘Animal Evolution: Inter-relationships of the Living Phyla’’ (’95; 2ndedition 2001).

This book evidently appeared at the rightmoment, and it soon became much used as a basefor morphological and embryological informationby colleagues working in the new field of ‘‘evolu-tionary developmental biology.’’ I was invited togive talks at a number of meetings with molecularbiologists, and I have felt a considerable mutualinspiration. This is probably one of the reasonswhy the St. Petersburg Society of Naturalists hasawarded me the Aleksander Kowalevsky Medal,for which I am deeply grateful.

C. NIELSEN62

LITERATURE CITED

Allen MJ. 1964. Embryological development of the syllid,Autolytus fasciatis (Bosc)(Class Polychaeta). Biol BullWoods Hole 127:187–205.

Akesson B. 1962. The embryology of Tomopteris helgolandica(Polychaeta). Acta Zool (Stockh) 43:135–199.

Akesson B. 1967. The embryology of the polychaete Eunicekobiensis. Acta Zool (Stockh) 48:141–192.

Anderson DT. 1966. The comparative embryology of thePolychaeta. Acta Zool (Stockh) 47:1–42.

Anderson DT. 1973. Embryology and Phylogeny of Annelidsand Arthropods (Int Ser Monogr Pure Appl Biol Zool 50).Oxford: Pergamon Press.

Anger K, Anger V, Hagmeier E. 1986. Laboratory studies onlarval growth of Polydora ligni, Polydora ciliata, andPygospio elegans (Polychaeta, Spionidae). Helgol Meeresun-ters 40:377–395.

Arendt D, Tessmar K, de Campos-Baptista M-IM, DorresteijnA, Wittbrodt J. 2002. Development of pigment-cup eyes inthe polychaete Platynereis dumerilii and evolutionaryconservation of larval eyes in Bilateria. Development129:1143–1154.

Arnolds WJA. 1982. A mutation with a maternal effect inLymnaea stagnalis L., affecting development of bilateralsymmetry and dorsoventrality and preventing gastrulation.Proceedings of the Koninklijke Nederlandse Akademie vanWetenschappen, Series C, Biological and Medical Sciences85:1–20.

Arnolds WJA, van den Biggelaar JAM, Verdonk NH. 1983.Spatial aspects of cell interactions involved in the determi-nation of dorsoventral polarity in equally cleaving gastro-pods and regulative abilities of their embryos, as studied bymicromere deletions in Lymnaea and Patella. Roux’s ArchsDev Biol 192:75–85.

Balfour FM. 1880. Larval forms: their nature, origin, andaffinities. Q J Microsc Sci NS 20:381–407.

Bartolomaeus T. 1989. Larvale Nierenorgane bei Lepidochitoncinereus (Polyplacophora) und Aeolidia papillosa (Gastro-poda). Zoomorphology 108:297–307.

Bartolomaeus T. 1992a. Ultrastructure of the photorecep-tors in certain larvae of the Annelida. Microfauna Mar7:191–214.

Bartolomaeus T. 1992b. Ultrastructure of the photoreceptorin the larvae of Lepidochiton cinereus (Mollusca, Polyplaco-phora) and Lacuna divaricata (Mollusca, Gastropoda).Microfauna Mar 7:215–236.

Bartolomaeus T. 1993. Different photoreceptors in juvenileOphelia rathkei (Annelida, Opheliida). Microfauna Mar8:99–114.

Bartolomaeus T. 1995. Ultrastructure of the protone-phridia in larval Magelona mirabilis (Spionida) andPectinaria auricoma (Terebellida): head kidneys in theground pattern of the Annelida. Microfauna Mar 10:117–141.

Bartolomaeus T. 1998. Head kidneys in hatchlings ofScoloplos armiger (Annelida: Orbiniida): implications forthe occurrence of protonephridia in lecithotrophic larvae. JMar Biol Ass UK 78:183–192.

Bartolomaeus T, Ax P. 1992. Protonephridia and metane-phridia - their relation within the Bilateria. Z Zool SystEvolutionsforsch 30:21–45.

Bayne B. 1965. Growth and delay of metamorphosis of thelarvae of Mytilus edulis. Ophelia 2:1–47.

Bellolio G, Lohrmann K, Dupre E. 1993. Larval morphology ofthe scallop Argopecten purpuratus as revealed by scanningelectron microscopy. Veliger 36:332–342.

Bleidorn C, Vogt L, Bartolomaeus T. 2003. New insights intopolychaete phylogeny (Annelida) inferred from 18S rDNAsequences. Mol Phyl Evol, in press.

Blumer MJF. 1996. Alterations of the eyes during ontogenesisin Aporrhais pespelecani (Mollusca, Caenogastropoda).Zoomorphology 116:123–131.

Blumer MJF. 1999. Development of a unique eye: photo-receptors of the pelagic predator Atlanta peroni (Gastro-poda, Heteropoda). Zoomorphology 119:81–91.

Boletzky S. 1988. Cephalopod development and evolutionaryconcepts. In: Wilbur KM, editor. The Mollusca, vol. 12. SanDiego: Academic Press. p 185–202.

Brandenburg J. 1966. Die Reusenformen der Cyrtocyten. ZoolBeitr, NF 12:345–417.

Brusca RC, Brusca GJ. 2003. Invertebrates, 2nd ed. Sunder-land, MA: Sinauer.

Buckland-Nicks J, Gibson G, Koss R. 2002. Phylum Mollusca:Gastropoda. In: Young CM, editor. Atlas of Marine Inverte-brate Larvae. San Diego: Academic Press. p 261–287.

Bullock TH, Horridge GA. 1965. Structure and Function inthe Nervous Systems of Invertebrates, 2 vols. San Francisco:Freemann.

Casse N, Devauchelle N, Le Pennec L. 1997. Embryonic shellformation in the scallop Pecten maximus (Linnaeus). Veliger40:350–358.

Chaffee C, Lindberg DR. 1986. Larval biology of EarlyCambrian molluscs: the implications of small body size.Bull Mar Sci 39:536–549.

Chaparro OR, Thompson RJ, Emerson CJ. 1999. The velarciliature in the brooded larva of the Chilean oyster Ostreachilensis (Philippi, 1845). Biol Bull Woods Hole 197:104–111.

Child CM. 1900. The early development of Arenicola andSternaspis. Arch EntwMech Org 9: 587–723.

Clement AC. 1967. The embryonic value of the micromeres inIlyanassa obsoleta, as determined by deletion experiments.I. The first quartet cells. J Exp Zool 166:77–88.

Clement AC. 1986a. The embryonic value of the micromeres inIlyanassa obsoleta, as determined by deletion experiments.II. The second quartet cells. Int J Invertebr Reprod Dev9:139–153.

Clement AC. 1986b. The embryonic value of the micromeresin Ilyanassa obsoleta, as determined by delation experi-ments. III. The third quartet cells and the mesoblast cell, 4d.Int J Invertebr Reprod Dev 9:155–168.

Conklin EG. 1897. The embryology of Crepidula. J Morph13:1–226.

Conklin EG. 1907. The embryology of Fulgur: a study of theinfluence of yolk on development. Proc Acad Nat Sci Philad59:320–359.

Costello DP. 1945. Experimental studies of germinal localiza-tion in Nereis. J Exp Zool 100:19–66.

Cragg SM. 1989. The ciliated rim of the velum of larvaeof Pecten maximus (Bivalvia: Pectinidae). J Moll Stud55:497–508.

Cragg SM. 1996. The phylogenetic significance of someanatomical features of bivalve larvae. In: Taylor J, editor.Origin and Evolutionary Radiation of the Mollusca.,Oxford:Oxford Univ. Press. p 371–380.

Cragg SM, Crisp DJ. 1991. The biology of scallop larvae. In:Shumway SE, editor. Biology, Ecology, and Aquaculture ofScallops. Amsterdam: Elsevier. p 75–132.


Croll RP, Voronezhskaya EE. 1996. Early elements ingastropod neurogenesis. Dev Biol 173:344–347.

Croll RP, Jackson DL, Voronezhskaya EE. 1997. Catechola-mine-containing cells in larval and postlarval bivalvemolluscs. Biol Bull Woods Hole 193:116–124.

Damen P, Dictus WJAG. 1994. Cell-lineage analysis of theprototroch of the gastropod mollusc Patella vulgata showsconditional specification of some trochoblasts. Roux’s ArchDev Biol 203:187–198.

Dautert E. 1929. Die Bildung der Keimblatter von Paludinavivipara. - Zool Jb, Anat 50:433–496.

Delsman HC. 1914. Entwicklungsgeschichte von Littorinaobtusata. Tijdschr Ned Dierk Vereen, 2. ser. 13:170–340, pls10–19.

Delsman HC. 1916. Eifurchung und Keimblattbildung beiScoloplos armiger O. F. Muller. Tijdschr Ned Dierk Vereen,2. ser. 14:283–498, pls 21–26.

Dickinson AJG, Nason J, Croll RP. 1999. Histochemicallocalization of FMRFamide, serotonin, and catecholaminesin embryonic Crepidula fornicata (Gastropoda, Prosobran-chia). Zoomorphology 119:49–62.

Dickinson AJG, Croll RP, Voronezhskaya EE. 2000. Develop-ment of embryonic cells containing serotonin, catechola-mines, and FMRFamide-related peptides in Aplysiacalifornica. Biol Bull Woods Hole 199:305–315.

Dictus WJAG, Damen P. 1997. Cell-lineage and clonal-contribution map of the trochophore larva of Patella vulgata(Mollusca). Mech Dev 62:213–226.

Dohle W. 1999. The ancestral cleavage pattern of theclitellates and its phylogenetic deviations. Hydrobiologia402:267–283.

Dohmen MR. 1992. Cell lineage in molluscan development.Microsc Res Tech 22:75–102.

Dorresteijn AWC, Fischer A. 1988. The process of earlydevelopment. Microfauna Mar 4:335–352.

Dorresteijn AWC. 1990. Quantitative analysis of cellulardifferentiation during early embryogenesis of Platynereisdumerilii. Roux’s Arch Dev Biol 199:14–30.

Dorresteijn AWC, O’Grady B, Fischer A, Porchet-Hennere E,Boilly-Marer Y. 1993. Molecular specification of cell lines inthe embryo of Platynereis (Annelida). Roux’s Arch Dev Biol202:260–269.

Drew GA. 1899a. Some observations on the habits, anatomy,and embryology of members of the Protobranchia. Anat Anz15:493–519.

Drew GA. 1899b. Yoldia limatula. Mem Biol Lab JohnsHopkins Univ 4:1–37, 5 pls.

Duda TF Jr, Palumbi SR. 1999. Developmental shifts andspecies selection in gastropods. Proc Nat Acad Sci USA96:10272–10277.

Eakin RM, Westfall JA. 1964. Further observations on thefine structure of some invertebrate eyes. Z Zellforsch 62:310–332.

Eernisse DJ. 1988. Reproductive patterns in six species ofLepidochitona (Mollusca: Polyplacophora) from the PacificCoast of North America. Biol Bull Woods Hole 174:287–302.

Eisig H. 1898. Zur Entwicklungsgeschichte der Capitelliden.Mitt Zool Sta Neapel 13:1–292, pls 1–9.

Emlet RB, Strathmann RR. 1994. Functional consequences ofsimple cilia in the mitraria of Oweniids (an anomalous larvaof an anomalous polychaete) and comparisons with otherlarvae. In: Wilson WH, Stricker SA, Shinn GL, editors.Reproduction and Development of Marine Invertebrates.Baltimore: Johns Hopkins University Press p 143–157.

Enders HE. 1909. A study of the life-history and habits ofChaetopterus variopedatus, Renier et Claparede. J Morph20:479–531, 3 pls.

Fernando W. 1931. The origin of the mesoderm in the gastro-pod Viviparus. Proc R Soc Lond B 107:381–390, pls 23–28.

Fioroni P. 1982. Larval organs, larvae, metamorphosis, andtypes of development of Mollusca - a comprehensive review.Zool Jb, Anat 108:375–420.

Focarelli R, Renieri T, Rosati F. 1988. Polarized site of spermentrance in the egg of a freshwater bivalve, Unio elongatu-lus. Dev Biol 127:443–451.

Focarelli R, Rosa D, Rosati F. 1990. Differentiation ofthe vitelline coat and polarized site of sperm entrance inthe egg of Unio elongatulus (Mollusca, Bivalvia). J Exp Zool254:88–96.

Fraipont J. 1887. Le genre Polygordius. Fauna Flora GolfNeapel 14:1–125, 16 pls.

Freeman G, Lundelius JW. 1992. Evolutionary implications ofthe mode of D quadrant specification in coelomates withspiral cleavage. J Evol Biol 5:205–247.

Friedrich S, Wanninger A, Bruckner M, Haszprunar G. 2002.Neurogenesis in the mossy chiton, Mopalia muscosa(Gould)(Polyplacophora): Evidence against molluscan me-tamerism. J Morph 253:109–117.

Giribet G, Wheeler W. 2002. On bivalve phylogeny: a high-level analysis of the Bivalvia (Mollusca) based on combinedmorphology and DNA sequence data. Invert Biol 121:271–324.

Giribet G, Distel DL, Polz M, Sterrer W, Wheeler WC. 2000.Triploblastic relationships with emphasis on the acoelo-mates and the position of Gnathostomulida, Cycliophora,Plathelminthes, and Chaetognatha: a combined approachof 18S rDNA sequences and morphology. Syst Biol 49:539–562.

Goldstein B, Freeman G. 1997. Axis specification in animaldevelopment. BioEssays 19:105–116.

Gotze E. 1938. Bau und Leben von Caecum glabrum(Montagu). Zool Jb, Syst 71:55–122.

Grobben K. 1908. Die systematische Einteilung des Tier-reichs. Verh Zool-Bot Ges Wien 58:491–511.

Gruner H-E. 1980. Einfuhrung. In: Gruner HE, editor. A.Kaestner’s Lehrbuch der speziellen Zoologie (4th ed.), 1.Band, 1. Teil, Stuttgart: Gustav Fischer. p 15–156.

Guerrier P. 1968. Origine et stabilite de la polarite animale-vegetative chez quelques Spiralia. Annls Embryol Morpho-gen 1:119–139.

Guerrier P. 1970. Les caracteres de la segmentation et de ladetermination de la polarite dorsoventrale dans le devel-oppement de quelques Spiralia. III. Pholas dactyluset Spisula subtruncata (Mollusques, Lamellibranches).J Embryol Exp Morph 23:667–692.

Gustafson RG, Lutz RA. 1992. Larval and early post-larvaldevelopment of the protobranch bivalve Solemya velum(Mollusca: Bivalvia). J Mar Biol Ass UK 72:383–402.

Gustafson RG, Reid RGB. 1986. Development of the perica-lymma larva of Solemya reidi (Bivalvia: Cryptodonta:Solemyidae) as revealed by light and electron microscopy.Mar Biol (Berl) 93:411–427.

Hadfield MG, Iaea DK. 1989. Velum of encapsulated veligersof Petaloconchus (Gastropoda), and the problem of re-evolution of planktotrophic larvae. Bull Mar Sci 45:377–386.

Hadfield MG, Meleshkevitch EA, Boudko DY. 2000. The apicalsensory organ of a gastropod veliger is a receptor forsettlement cues. Biol Bull Woods Hole 198:67–76.

C. NIELSEN64

Halanych KM, Dahlgren TG, McHugh D. 2002. Unsegmentedannelids? Possible origins of four lophotrochozoan wormtaxa. Integ Comp Biol 42:678–684.

Hannerz L. 1956. Larval development of the polychaetefamilies Spionidae Sars, Disomidae Mesnil, and Poecilo-chaetidae n.fam. in the Gullmar Fjord. Zool Bidr Upps 31:1–204.

Hansen B. 1993. Aspects of feeding, growth, and stagedevelopment by trochophora larvae of the boreal polychaeteMediomastus fragile (Rasmussen)(Capitellidae). J Exp MarBiol Ecol 166:273–288.

Haszprunar G, Schaefer K. 1997. Monoplacophora. In:Harrison FW, editor. Microscopic Anatomy of Invertebrates,vol. 6B. New York: Wiley-Liss. p 415–457.

Haszprunar G, Ruthensteiner B. 2000. Microanatomy andultrastructure of the protonephridial system of the larva ofthe limpet, Patella vulgata L. (Mollusca, Patellogastropoda).J Submicrosc Cytol Pathol 32:59–67.

Haszprunar G, Salvini-Plawen L, Rieger R. 1995. Larvalplanktotrophy - a primitive trait in the Bilateria? Acta Zool(Stockh) 76:141–154.

Hatschek B. 1878. Studien uber Entwicklungsgeschichte derAnneliden. Arb Zool Inst Univ Wien 1:277–404, pls 23–30.

Hatschek B. 1880. Ueber Entwicklungsgeschichte vonEchiurus und die systematische Stellung der Echiuridae(Gephyrei chaetiferi). Arb Zool Inst Univ Wien 3:45–78,pls 4–6.

Hatschek B. 1891. Lehrbuch der Zoologie, 3. Lieferung Jena:Gustav Fischer. (p 305–432).

Hay-Schmidt A. 1995. The larval nervous system of Poly-gordius lacteus Schneider, 1868 (Polygordiidae, Polychaeta):immunocytochemical data. Acta Zool (Stockh) 76:121–140.

Heath H. 1899. The development of Ischnochiton. Zool Jb,Anat 12:567–656, pls 31–35.

Heimler W. 1988. Larvae. Microfauna Mar 4:353–371.Henry JJ, Martindale MQ. 1987. The organizing role of the D

quadrant as revealed through the phenomenon of twinningin the polychaete Chætopterus variopedatus. Roux’s ArchDev Biol 196:499–510.

Hermans CO. 1978. Metamorphosis in the opheliid polychaeteArmandia brevis. In: Chia F-S, Rice ME, editors. Settlementand Metamorphosis of Marine Invertebrate Larvae. NewYork: Elsevier. p 113–126.

Hessling R. 2002. Metameric organisation of the nervoussystem in developmental stages of Urechis caupo (Echiura)and its phylogenetic implications. Zoomorphology 121:221–234.

Hessling R, Westheide W. 2002. Are Echiura derived from asegmented ancestor? Immunohistochemical analysis of thenervous system in developmental stages of Bonellia viridis.J Morph 252:100–113.

Hickman CS. 1999. Larvae in invertebrate development andevolution. In: Hall BK, Wake MH, editors. The Origin andEvolution of Larval Forms. San Diego: Academic Press.p 21–59.

Huxley TH. 1853. Lacunularia socialis. A contribution to theanatomy and physiology of the Rotifera. Q J Micr Sci (TransMicr Soc Lond, N.S. 1) 1:1–19, pls 1–3.

Hyman LH. 1951. The Invertebrates, vol. 1. New York:McGraw-Hill.

Hyman LH. 1967. The Invertebrates, vol. 6. New York:McGraw-Hill.

Irvine SQ, Chaga O, Martindale MQ. 1999. Larval ontogeneticstages of Chaetopterus: developmental heterochrony in the

evolution of chaetopterid polychaetes. Biol Bull Woods Hole197:319–331.

Ivanova-Kazas OM. 1985. The origin and phylogeneticsignificance of the trochophoran larvae. 2. Evolutionarysignificance of the larvae of coelomate worms and molluscs.Zool Zh 64:650–660 (In Russian, English summary).

Ivanova-Kazas OM. 1987. Origin, evolution, and phylogeneticsignificance of ciliated larvae. Zool Zh 66:325–338 (InRussian, English summary).

Ivanova-Kazas OM, Ivanov AV. 1988. Trochaea theory andphylogenetic significance of ciliate larvae. Soviet J Mar Biol13:67–80.

Jacob MH. 1984. Neurogenesis in Aplysia californica resem-bles nervous system formation in vertebrates. J Neurosci4:1225–1239.

Jenner RA. 2003. Unleashing the force of cladistics? Metazoanphylogenetics and hypothesis testing. Integ Comp Biol 43:in press.

Jagersten G. 1972. Evolution of the Metazoan Life Cycle.London: Academic Press.

Kempf SC, Page LR, Pires A. 1997. Development of serotonin-like immunoreactivity in the embryos and larvae ofnudibranch mollusks with emphasis on the structure andpossible function of the apical sensory organ. J Comp Neurol386:507–528.

Korn H. 1960. Das larvale Nervensystem von PectinariaLamarck und Nephthys Cuvier (Annelida, Polychaeta). ZoolJb, Anat 78:427–456.

Kowalevsky A. 1883a. Embryogenie du Chiton polii (Philippi).Annls Mus Hist Nat Marseille, Zool 1(5):1–46, 8 pls.

Kowalevsky A. 1883b. Etude sur l’embryogenie du Dentale.Annls Mus Hist Nat Marseille, Zool 1(7):1–54, 8 pls.

Lacalli TC. 1981. Structure and development of the apicalorgan in trochophores of Spirobranchus polycerus, Phyllo-doce maculata, and Phyllodoce mucosa (Polychaeta). Proc RSoc Lond B 212:381–402.

Lacalli TC. 1984. Structure and organization of the nervoussystem in the trochophore larva of Spirobranchus. PhilTrans R Soc B 306:79–135.

Lacalli TC. 1988. Structural correlates of photoresponse introchophore larva. Can J Zool 66:1004–1006.

Lambert JD, Nagy LM. 2001. MAPK signaling by the Dquadrant embryonic organizer of the mollusc Ilyanassaobsoleta. Development 128:45–56.

Lankester ER. 1874. Observations on the development of thepond-snail (Lymnæus stagnalis), and on the early stages ofother Mollusca. Q J Microsc Sci, NS 14:365–391, pls 16–17.

Lartillot N, Lespinet O, Vervoort M, Adoutte A. 2002.Expression pattern of Brachyury in the mollusc Patellavulgata suggests a conserved role in the establishment ofthe AP axis in Bilateria. Development 129:1411–1421.

Lieberman BS, Allmon WD, Eldredge N. 1993. Levels ofselection and macroevolutionary patterns in the turritellidgastropods. Paleobiology 19:205–215.

Lillie FR. 1895. The embryology of the Unionidae. J Morph10:1–100, pls 1–6.

Lillie FR. 1901. Organization of the egg of Unio, based on astudy of its maturation, fertilization, and cleavage. J Morph17:227–292, pls 24–27.

Lillie FR. 1906. Observations and experiments concerning theelementary phenomena of embryonic development inChætopterus. J Exp Zool 3:153–268, 1 pl.

Lin M-F, Leise EM. 1996a. Gangliogenesis in the prosobranchgastropod Ilyanassa obsoleta. J Comp Neurol 374:180–193.


Lin M-F, Leise EM 1996b. NADPH-diaphorase activitychanges during gangliogenesis and metamorphosis in thegastropod mollusc Ilyanassa obsoleta. J Comp Neurol374:194–203.

Longo FJ. 1983. Meiotic maturation and fertilization. In:Wilbur KM, editor. The Mollusca, vol. 3. New York:Academic Press. p 49–89.

Loven S. 1840. Iakttagelse ofver metamorfos hos en Annelid.K Svenska Vetenskapsakad Handl 1840:93–98, pl.2.

Luetjens CM, Dorresteijn AWC. 1998. The site of fertilisationdetermines the dorsoventral polarity but not chirality in thezebra mussel embryo. Zygote 6:125–135.

Marsden JR, Hsieh J. 1987. Ultrastructure of the eyespot inthree polychaete trochophoree larvae (Annelida). Zoomor-phology 106:361–368.

Mead AD. 1897. The early development of marine annelids.J Morph 13:227–326.

Meisenheimer J. 1901. Entwicklungsgeschichte von Dreissen-sia polymorpha Pall. Z Wiss Zool 69:1–137, pls. 1–13.

Miner BG, Sanford E, Strathmann RR, Pernet B, Emlet RE.1999. Functional and evolutionary implications of opposedbands, big mouths, and extensive oral ciliation in larvalopheliids and echiurids (Annelida). Biol Bull Woods Hole197:14–25.

Moor B. 1973. Zur fruhen Furchung des Eies von Littorinalittorea L. (Gastropoda, Prosobranchia). Zool Jb, Anat91:546–573.

Morgan TH, Tyler A. 1930. The point of entrance of thespermatozoon in relation to the orientation of the embryo ineggs with spiral cleavage. Biol Bull Woods Hole 58:59–73.

Morgan TH, Tyler A. 1938. The relation between entrancepoint of the spermatozoon and bilaterality of the egg ofChaetopterus. Biol Bull Woods Hole 74:401–402.

Morrill JB, Blair CA, Larsen WJ. 1973. Regulative develop-ment in the pulmonate gastropod, Lymnaea palustris, asdetermined by blastomere deletion experiments. J Exp Zool183:47–56.

Morse MP, Zardus JD. 1997. Bivalvia. In: Harrison FW, editor.Microscopic Anatomy of Invertebrates, vol. 6A. New York:Wiley-Liss. p 7–118.

Nelson JA. 1904. The early development of Dinophilus: astudy in cell-lineage. Proc Acad Nat Sci Philad 56:687–737,pls 43–48.

Nielsen C. 1964. Studies on Danish Entoprocta. Ophelia 1:1–76.

Nielsen C. 1966. On the life-cycle of some Loxosomatidae(Entoprocta). Ophelia 3:221–247.

Nielsen C. 1971. Entoproct life-cycles and the entoproct/ectoproct relationship. Ophelia 9:209–341.

Nielsen C. 1979. Larval ciliary bands and metazoan phylo-geny. Fortschr Zool Syst Evolutionsforsch 1:178–184.

Nielsen C. 1985. Animal phylogeny in the light of the trochaeatheory. Biol J Linn Soc 25:243–299.

Nielsen C. 1987. Structure and function of metazoan ciliarybands and their phylogenetic significance. Acta Zool(Stockh) 68:205–262.

Nielsen C. 1989. Entoprocts. Synopses British Fauna, N.S.41:1–131.

Nielsen C. 1995. Animal Evolution: Interrelationships of theLiving Phyla. Oxford: Oxford University Press.

Nielsen C. 1998. Origin and evolution of animal life cycles.Biol Rev 73:125–155.

Nielsen C. 2001. Animal Evolution: Interrelationships of theLiving Phyla, 2nd ed. Oxford: Oxford University Press.

Novikoff AB. 1938. Embryonic determination in the annelid,Sabellaria vulgaris. II. Transplanation of polar lobes andblastomeres as a test of their inducing capacities. Biol BullWoods Hole 74:211–234.

Okusu A. 2002. Embryogenesis and development of Epimeniababai (Mollusca Neomeniomorpha). Biol Bull Woods Hole203:87–103.

Page LR. 1992. New interpretation of a nudibranch centralnervous system based on ultrastructural analysis of neuro-development in Melibe leonina. I. Cerebral and visceral loopganglia. Biol Bull Woods Hole 182:348–365.

Page LR. 1994. The ancestral gastropod larval form is bestapproximated by hatching-stage opisthobranch larvae:evidence from comparative developmental studies. In:Wilson WH Jr, Stricker SA, Shinn GL, editors. Reproduc-tion and Development of Marine Invertebrates. Baltimore:The Johns Hopkins University Press. p 206–223.

Page LR. 2002. Apical sensory organ in larvae of thepatellogastropod Tectura scutum. Biol Bull Woods Hole202:6–22.

Page LR, Parries SC. 2000. Comparative study of the apicalganglion in planktotrophic caenogastropod larvae: ultra-structure and immunoreactivity to serotonin. J CompNeurol 418:383–401.

Page LR, Pedersen RVK. 1998. Transformation of phyto-planktivorous larvae into predatory carnivores during thedevelopment of Polynices lewisii (Mollusca, Caenogastropo-da). Invert Biol 117:208–220.

Pearse JS. 1979. Polyplacophora. In: Giese AC, Pearse JS,editors. Reproduction of Marine Inverebrates, vol. 5. NewYork: Academic Press. p 27–85.

Penners A. 1926. Experimentelle Untersuchungen zumDeterminationsproblem am Keim von Tubifex rivulorumLam. II. Die Entwicklung teilweise abgetoteter Keime. ZWiss Zool 127:1–140.

Pernet B, Qian P-Y, Rouse G, Young CM, Eckelbarger KJ.2002. Phylum Annelida: Polychaeta. In: Young CM, editor.Atlas of Marine Invertebrate Larvae. San Diego: AcademicPress. p 209–243.

Peterson KJ, Eernisse DJ. 2001. Animal phylogeny and theancestry of bilaterians: inferences from morphology and 18SrDNA sequences. Evol Dev 3:170–205.

Phillips NE, Pernet B. 1996. Capture of large particles bysuspension feeding scaleworm larvae (Polychaeta: Polynoi-dae). Biol Bull Woods Hole 191:199–208.

Pruvot G. 1890. Sur le developpement d’un Solenogastre. CRHebd Seanc Acad Sci Paris 111:689–695.

Raineri M. 1995. Is a mollusc an evolved bent metatrocho-phore? A histochemical investigation of neurogenesis inMytilus (Mollusca: Bivalvia). J Mar Biol Ass UK 75:571–592.

Raineri M. 2000. Early neurogenesis pattern in Patellacoerulea (Patellogastropoda) and its possible phylogeneticimplications. Malacologia 42:131–148.

Raineri M, Ospovat M. 1994. The initial development ofgangliar rudiments in a posterior position in Mytilusgalloprovincialis (Mollusca: Bivalvia). J Mar Biol Ass UK74:73–77.

Rasmussen E. 1973. Systematics and ecology of the Isefjordmarine fauna. Ophelia 11:1–495.

Raven CP. 1966. Morphogenesis: The Analysis of MolluscanDevelopment. 2nd ed. Oxford: Pergamon Press.

Reid DG. 1989. The comparative morphology, phylogeny, andevolution of the gastropod family Littorinidae. Phil Trans RSoc B 324:1–110.

C. NIELSEN66

Render JA. 1983. The second polar lobe of the Sabellariacementarium embryo plays an inhibitory role in apical tuftformation. Roux’s Arch Dev Biol 192:120–129.

Render J. 1997. Cell fate maps in the Ilyanassa obsoletaembryo beyond the third division. Dev Biol 189:301–310.

Reverberi G. 1971. Dentalium. In: Reverberi G, editor:Experimental Embryology of Marine and FreshwaterInvertebrates. Amsterdam: North-Holland. p 248–264.

Rhode B. 1992. Development and differentiation of the eye inPlatynereis dumerilii (Annelida, Polychaeta). J Morph212:71–85.

Rhode B. 1993. Larval and adult eyes in Capitella sp. J Morph217:327–335.

Riisgard HU, Nielsen C, Larsen PS. 2000. Downstreamcollecting in ciliary suspension feeders: the catch-upprinciple. Mar Ecol Prog Ser 207:33–51.

Robert A. 1903. Recherches sur le developpement des troques.Archs Zool Exp Gen 10:269–538, pls 12–42.

Rosati F, Capone A, della Giovampaola C, Brettoni C, FocarelliR. 2000. Sperm-egg interaction at fertilization: glycans asrecognition signals. Int J Dev Biol 44:609–618.

Rosen MD, Stasek CR, Hermans CO. 1978. The ultrastructureand evolutionary significance of the cerebral ocelli ofMytilus edulis, the bay mussel. Veliger 21:10–18, 4 pls.

Rosen MD, Stasek CR, Hermans CO. 1979. The ultrastructureand evolutionary significance of the ocelli in the larva ofKatharina tunicata. Veliger 22:173–178, 3 pls.

Rouse GW. 1999. Trochophore concepts: ciliary bands and theevolution of larvae in spiralian Metazoa. Biol J Linn Soc66:411–464.

Rouse GW, Fauchald K. 1995. The articulation of annelids.Zool Scr 24:269–301.

Rouse GW, Fauchald K. 1997. Cladistics and polychaetes.Zool Scr 26:139–204.

Rullier F. 1955. Developpement du Serpulien Mercierellaenigmatica Fauvel. Vie Milieu 6:225–240.

Rullier F. 1960. Morphologie et developpement du Spionidae(Annelide polychete) Polydora (Boccardia) redeki Horst.Cah Biol Mar 1:231–244.

Ruthensteiner B, Wanninger A, Haszprunar G. 2001. Theprotonephridial system of the tusk shell, Antalis entalis(Mollusca, Scaphopoda). Zoomorphology 121:19–26.

Salvini-Plawen LV. 1972. Zur Morphologie und Phylogenieder Mollusken: Die Beziehungen der Caudofoveata und derSolenogastres als Aculifera, als Mollusca und als Spiralia.Z Wiss Zool 184:205–394.

Salvini-Plawen LV. 1980. Was ist eine Trochophora? EineAnalyse der Larventypen mariner Protostomier. Zool Jb,Anat 103:389–423.

Salvini-Plawen LV. 1985. Early evolution and the primitivegroups. In: Wilbur KM, editor. The Mollusca, vol. 10.Orlando: Academic Press. p 59–150.

Schaefer K, Ruthensteiner B. 2001 The cephalic sensory organin pelagic and intracapsular larvae of the primitiveopisthobranch genus Haminoea (Mollusca: Gastropoda).Zool Anz 240:69–82.

Scheltema AH. 1993. Aplacophora as progenetic aculiferansand the coelomate origin of mollusks as the sister taxon ofSipuncula. Biol Bull Woods Hole 184:57–78.

Scheltema AH, Ivanov DL. 2002. An aplacophoran postlarvawith iterateddorsal groups of spicules and skeletal simila-rities to Paleozoic fossils. Invert Biol 121:1–10.

Segrove F. 1941. The development of the serpulid Pomatocerostriqueter L. Q J Microsc Sci, NS 82:467–540.

Semper C. 1872. Zoologische Aphorismen. III. Trochosphaeraaequatorialis, das Kugelraderthier der Philippinen. Z WissZool 22:305–322, pls 22–24.

Semper C. 1876. Die Vervandtschaftsbeziehungen der geglie-derten Thiere. III. Strobilation und Segmentation. EinVersuch zur Feststellung specieller Homologien zwischenVertebraten, Anneliden und Arthropoden. Arb Zool-ZootomInst Wurzburg 3:115–404, pls 5–15.

Shankland M, Savage MR. 1997. Annelids, the segmentedworms. In: Gilbert SF, Raunio AM, editors. Embryology.Constructing the Organism. Sunderland: Sinauer Associ-ates. p 219–235.

Shankland M, Seaver EC. 2000. Evolution of the bilaterianbody plan: what have we learned from annelids? Proc NatnAcad Sci USA 97:4434–4437.

Strathmann RR, Eernisse DJ. 1994. What molecular phylo-genies tell us about the evolution of larval forms. Am Zool34:502–512.

Strathmann RR, Jahn TL, Fonseca JR. 1972. Suspensionfeeding by marine invertebrate larvae: Clearance of parti-cles by ciliated bands of a rotifer, pluteus, and trochophore.Biol Bull Woods Hole 142:505–519.

Struck T, Hessling R, Purschke G. 2002. The phylogeneticposition of the Aeolosomatidae and Parergodrilidae, twoenigmatic oligochaete-like taxa of the ‘Polychaeta’, based onmolecular data from 18S rDNA sequences. J Zool SystEvolut Res 40:155–163.

Sweet HC. 1998. Specification of first quartet micromeres inIlyanassa involves inherited factors and position withrespect to the inducing D macromere. Development125:4033–4044.

Tardy J, Dongard S. 1993. Le complexe apical de la veligere duRuditapes philippinarum (Adams et Reeve, 1850) MollusqueBivalve Veneride. CR Seanc Acad Sci, Paris, 3 Serie,316:177–184.

Thavaradhara K, Leise E. 2001. Localization of nitric oxidesynthase-like immunoreactivity in the developing nervoussystem of Ilyanassa. J Neurocytol 30:449–456.

Thompson TE. 1960. The development of Neomenia carinataTullberg (Mollusca Aplacophora). Proc R Soc Lond B153:263–278.

Thorson G. 1946. Reproduction and larval development ofDanish marine bottom invertebrates, with special referenceto the planktonic larvae in the Sound (Øresund). MeddrKommn Danm Fisk- Havunders Plankton 4:1–523.

Tomlinson SG. 1987. Intermediate stages in the embryonicdevelopment of the gastropod Ilyanassa obsoleta: a scanningelectron microscopic study. Int J Invertebr Reprod Dev12:253–280.

Torrence SA, Stuart DK. 1986. Gangliogenesis in leechembryos: migration of neural precursor cells. J Neurosci6:2736–2746.

Torrey JC. 1903. The early embryology of Thalassema mellita(Conn). Ann NY Acad Sci 14:165–246.

Treadwell AL. 1901. Cytogeny of Podarke obscura Verrill.J Morph 17:399–486.

Tzetlin AB. 1998. Giant pelagic larvae of Phyllodocidae(Polychaeta, Annelida). J Morph 238:93–107.

van den Biggelaar JAM. 1977. Development of dorsoventralpolarity and mesentoblast determination in Patella vulgata.J Morph 154:157–186.

van den Biggelaar JAM. 1993. Cleavage pattern in embryos ofHaliotis tuberculata (Archaeogastropoda) and gastropodphylogeny. J Morph 216:121–139.


van den Biggelaar JAM. 1996. Cleavage pattern and mesento-blast formation in Acanthochiton crinitus (Polyplacophora,Mollusca). Dev Biol 174:423–430.

van den Biggelaar JAM, Guerrier P. 1983. Origin of spatialorganization. In: Wilbur KM, editor. The Mollusca, vol. 3.New York: Academic Press. p 179–213.

van den Biggelaar JAM, Haszprunar G. 1996. Cleavagepatterns and mesentoblast formation in the Gastropoda:an evolutionary perspective. Evolution 50:1520–1540.

van den Biggelaar JAM, Edsinger-Gonzales E, Schram FR.2002. The improbability of dorso-ventral axis inversionduring animal evolution as presumed by Geoffroy SaintHilaire. Contr Zool 71:29–36.

van Dongen CAM, Geilenkirchen WLM. 1974. The develop-ment of Dentalium with special reference to the significanceof the polar lobe. I-III. Division chronology and developmentof the cell pattern in Dentalium dentale (Scaphopoda). ProcK Ned Akad Wet 77:57–100.

Verdonk NH. 1979. Symmetry and asymmetry in theembryonic development of molluscs. In: van der Spoel S,van Bruggen AC, Lever J, editors. Pathways in Malacology.Utrecht: Bohn, Scheltema & Holkema. p 25–45.

Verdonk NH, van den Biggelaar JAM. 1983. Early develop-ment and the formation of the germ layers. In: Wilbur KM,editor. The Mollusca, vol. 3. New York: Academic Press.p 91–122.

Voronezhskaya EE, Tyurin SA, Nezlin LP. 2002. Neuronaldevelopment in larval chiton Ischnochiton hakodadensis(Mollusca: Polyplacophora). J Comp Neurol 444:25–38.

Voronezhskaya EE, Tsitrin EB, Nezlin LP. 2003. Neuronaldevelopment in larval polychaete Phyllodoce maculata(Phyllodocidea). J Comp Neurol 455:299–309.

Waller TR. 1981. Functional morphology and development ofveliger larvae of the European oyster, Ostrea edulis Linne.Smithson Contr Zool 328:1–70.

Wanninger A, Haszprunar G. 2001. The expression of anengrailed protein during embryonic shell formation of thetusk-shell, Antalis entalis (Mollusca, Scaphopoda). Evol Dev3:312–321.

Wanninger A, Haszprunar G. 2002. Chiton myogenesis:perspectives for the development and evolution of larvaland adult muscle systems in molluscs. J Morph 251:103–113.

Wanninger A, Haszprunar G. 2003. The development of theserotonergic and FMRF-amidergic nervous system in Anta-lis entalis (Mollusca, Scaphopoda). Zoomorphology 122:77–85.

Wanninger A, Ruthensteiner B, Lobenwein S, Salvenmoser W,Dictus WJAG, Haszprunar G. 1999. Development of the

musculature of the limpet Patella (Mollusca, Patellogastro-poda). Dev Genes Evol 209:226–238.

Werner B. 1953. Beobachtungen uber den Nahrungserwerbund die Metamorphose der Metatrochophora von Chaetop-terus variopedatus Renier u. Claparede (Polychaeta Seden-taria). Helgolander Wiss Meeresunters 4:225–238.

Werner B. 1955. Uber die Anatomie, die Entwicklung undBiologie des Veligers und der Veliconcha von Crepidulafornicata L. (Gastropoda, Prosobranchia). Helgolander WissMeeresunters 5:169–217.

Westheide W, McHugh D, Purschke G, Rouse G. 1999.Systematization of the Annelida: different approaches.Hydrobiologia 402:291–307.

Wierzejski A. 1905. Embryologie von Physa fontinalis L.Z Wiss Zool 83:502–706, pls 18–27.

Wilson EB. 1892. The cell lineage of Nereis. J Morph 6:361–480, pls 13–20.

Wilson EB. 1898. Considerations on cell-lineage and ancestralreminiscence. Ann NY Acad Sci 11:1–27.

Wilson EB. 1904a. Experimental studies in germinal localiza-tion. II. Experiments on cleavage-mosaic in Patella andDentalium. J Exp Zool 1:197–268.

Wilson EB. 1904b. Mosaic development in the annelid egg.Science 20:748–750.

Wilson DP. 1932. On the mitraria larva of Owenia fusiformisDelle Chiaje. Phil Trans R Soc B 221:231–334.

Wilson DP. 1939. The larval development of Ophelia bicornisSavigny. J Mar Biol Ass UK 27:540–553.

Woltereck R. 1902. Trochophora-Studien I. Histologieder Larve und die Entstehung des Annelids beiden Polygordius-Arten der Nordsee. Zoologica (Stuttg)13:1–71.

Woltereck R. 1904. Beitrage zur praktischen Analyse derPolygordius-Entwicklung nach dem ‘‘Nordsee-’’ und dem‘‘Mittelmeer-Typus.’’ Arch EntwMech Org 18:377–403.

Wray GA. 1995. Evolution of larvae and developmental modes.In: McEdward L, editor. Ecology of Marine InvertebrateLarvae. Boca Raton, FL: CRC Press. p 413–447.

Young C, Vazquez E, Metaxas A, Tyler PA. 1996. Embryologyof vestimentiferan tube worms from deep-sea methane/sulphide seeps. Nature 381: 514–516.

Zardus JD, Morse MP. 1998. Embryogenesis, morphology, andultrastructure of the pericalymma larva of Acila castrensis(Bivalvia: Protobranchia: Nuculoida). Invert Biol 117:221–44.

Zardus JD, Martel AL. 2002. Phylum Mollusca: Bivalvia. In:Young CM, editor. Atlas of Marine Invertebrate Larvae. SanDiego: Academic Press. p 289–325.

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Trochophora Larvae Cell-Lineages Ciliary Bands