Echinodermata P

Echinoderms are secondarily radially symmetric deuterostomes whose ancestors were

bilaterally symmetric. The adult radial symmetry is pentamerous with body parts occurring in

fives or multiples thereof. All echinoderms are marine and benthic. About 6000 recent species

are known but the fossil record includes 13,000 extinct species.

An important echinoderm trait is the water vascular system that in most groups functions

in support of locomotory tube feet but is also important in gas exchange, excretion, and feeding.

The body wall includes a thick connective tissue dermis in which calcareous ossicles (little

bones) are almost always an important component. These ossicles make up an endoskeleton

which assumes different forms in different taxa. In most echinoderms calcareous spines of

various sizes and shapes arise from the dermis and extend from the body surface and are alluded

to by the name echinoderm (= spiny skin). The connective tissue is mutable and its consistency

is under nervous control.

Excretion in echinoderms is accomplished by simple diffusion of metabolic wastes

(ammonia) across thin permeable regions of the body wall. A variety of gas exchange structures,

including the tube feet, is found in various echinoderms. A hemal system is present but its role

in transport is still poorly understood and the chief transport system is the circulating fluid of the

various coelomic compartments. Echinoderms are gonochoric and fertilization is usually

external.

Asteroidea C

Asteroids are the sea stars, which are the best known echinoderms. Sea stars usually

have five arms, but sometimes more, radiating from a central disk. The ossicles of the body wall

are rodlike and respiration is with the tube feet and papulae. Each arm has an eyespot at its tip. A

pair of large pyloric ceca and a pair of gonads are present in each arm. About 1500 recent

species are known.

External Anatomy

The body is divided into a central disk from which radiate five arms. The principal body

axis, and the axis of symmetry, is the short oral-aboral axis, which passes vertically through the

center of the disk. The animal's pale lower side is the oral surface and structures on this side are

said to be oral. The dark upper side is the aboral surface and structures on this side are aboral.

Structures remote from the axis are said to be peripheral whereas those near the axis are

central. In radially symmetrical animals anterior-posterior, dorsal-ventral, right-left are

irrelevant and have no meaning.

Aboral Surface

Find the calcareous, sometimes orange madreporite on the aboral surface of the disk.

Examine it with the high power of the dissecting microscope and note its grooved surface.

Numerous microscopic pores in the bottoms of the grooves open into canals (stone canal and

axial canal) of the internal water vascular system.

Orient the star with the aboral side up and with the madreporite close to you. The arm on

the left of the madreporite is arm I, arm II is to the right of the madreporite, and the remaining

arms are numbered sequentially moving counterclockwise around the star. A radial axis passing

from the center of the disk outward along the midline of any arm is a radius, or ambulacral axis,

of which there are five. Any axis bisecting the angle between any two adjacent arms is an

interambulacral axis, or interradial axis, and there are five of these also. One interambulacral

axis passes through the madreporite.

Figure 1 Aboral view of Asterias.

On the aboral surface notice the numerous small fixed spines, so-called because they are

fixed in position and cannot move. These spines are extensions of the calcareous endoskeleton

in the body wall. Gently push one of the spines with the tip of a needle to see if it moves. Look

closely at the spines with the highest magnification of the dissecting microscope and confirm that

they are indeed internal and are covered by a thin layer of living tissue, the epidermis. Each

spine is surrounded by a circle of short-stemmed, white pedicellariae (singular: pedicellaria.

Pedicellariae have an endoskeleton of ossicles.

Remove several pedicellariae with your fine forceps and place them in a drop of bleach on a

microscope slide. Wait a few minutes for the organic tissue to be oxidized and then place a

coverslip over the drop. Examine it with the compound microscope and look for the jaw-like

ossicles. These pedicellariae contain three ossicles. One is a short basal piece in the stalk of the

pedicellaria whereas the other two support the two jaws (Fig 28-10). Tiny muscles extend

between these ossicles to operate the jaws but these will have been removed by the bleach.

Examine an ossicle with 400X to see the numerous pores that perforate it. If there is too much

soft tissue remaining, the pores, or even the ossicles themselves, may not be visible. Try looking

at several ossicles with carefully adjusted light if necessary to find pores. Such pores are

characteristic of echinoderm ossicles and prevent the spread of cracks.

Between the spines are many soft, thin-walled, translucent, fingerlike papulae. Papulae

are thin-walled diverticula of the coelom through the body wall and are its respiratory organs.

The ciliated peritoneum generates a bidirectional flow of fluid into and out of the papulae. The

papulae are muscular and can be retracted into the surface of the body wall. They may be

retracted and inconspicuous in preserved specimens. If you have a living specimen touch a

papula with the microneedle and observe its response.

The anus is located near the center of the aboral surface but is almost impossible to

demonstrate externally. It is surrounded by a palisade of tiny ossicles, much smaller than the

spines that stud the surface of the disk and is in an area free of papulae.

Oral Surface

Turn the animal over and study the oral surface. Find the large mouth in the center of the

disk, surrounded by the thin peristomial membrane. The yellowish-orange curtain-like folds of

the cardiac stomach may be visible inside the mouth.

Five deep ambulacral grooves radiate outward from the mouth, one along the midline of

the oral surface of each arm. Each groove lies on an ambulacral axis. The numerous soft,

tubular structures projecting into the groove from either side are the tube feet, or podia. Two

rows of tube feet are present on each side of the groove. The tube feet of Asterias bear suckers

at their distal ends. Note the rows of long, flattened movable spines on each side of the

ambulacral groove. The word ambulacrum is Latin for "covered way," an apt name as these

spines are used to cover the groove to protect the tube feet.

Look at the tip of one of the arms. As is usual in radially symmetrical animals, the

sensory structures are arrayed around the periphery, which in sea stars is the tips of the arms.

Several long, narrow sensory tube feet extend from the tip of each arm. These are easily seen in

living specimens but contract and become inconspicuous in preserved material. They have

chemo- and mechanoreceptors. At the tip of the arm is a small circle of short, blunt movable

spines that are not associated with pedicellariae. These spines surround a small, pale red or

yellow eyespot. The eyespot is on the oral surface of the arm, almost at the tip.

Internal Anatomy

Preserved specimens should remain in tapwater.

Use a robust pair of scissors to cut the end from arm III about 2 cm from its tip as

indicated in Figure 1. Insert the sharp point of the scissors into the opening and cut along the

side of the arm until you reach the disk as indicated by the dotted lines on the figure. Make a

second cut, similar to the first, on the other side of the same arm. Do not lift the aboral body wall

off the disk yet. Extend the cuts around the margin of the disk and across the bases of the other

arms but DO NOT cut between the madreporite and the edge of the disk. Do not damage the

madreporite or structures lying inside the body wall.

Gently lift the aboral body wall slightly and with a blunt probe or teasing needle

carefully free it from the underlying tissues to which it is connected by mesenteries and. Do this

without damaging the soft tissues. Lift the body wall of the disk enough to see beneath it and

look on its inside surface to find the point at which the inconspicuous intestine enters it to reach

the anus. The small, lobed, olive-green (in life) rectal cecum surrounds the intestine and

obscures its junction with the body wall.

Figure 2. Asterias in aboral view with the body wall removed from the disk and two arms.

The pyloric ceca have been removed from arm V.

Once you have found the cecum, free it from the body wall so it remains with the rest of

the viscera. Cut across the aboral disk so that the madreporite remains intact (Fig 1). Remove

the now free portions of the body wall. The intestine will probably be destroyed by this

procedure. Leave the organs of the body cavity intact. Set the body wall aside but keep it

immersed.

Make a preliminary examination of the body cavity and its organs. Identify the major

organs now so you can use them as landmarks later. The space you have exposed is the

perivisceral coelom. Most of the interior of the central disk is occupied by the cardiac

stomach. It is a large mass of thin orangish tissue. It is highly extensible and can accommodate

large prey when extended outside the body. Two large, brownish, greenish, or creamy-white (in

life) pyloric ceca (= digestive ceca, hepatic ceca, digestive glands) occupy most of the aboral

half of the arms.

Two gonads lie in the oral half of the each arm hidden by the pyloric ceca. Their size

depends on reproductive condition and they may be very small or absent in immature or

reproductively inactive specimens.

Lift the pyloric ceca and gonads to reveal the floor of the arm. Locate the conspicuous,

raised ambulacral ridge running lengthwise along the middle of the arm. It is the internal

manifestation of the ambulacral groove you saw on the outside of the arm. It is formed of

sequentially arranged ambulacral ossicles in the body wall. The divisions between adjacent

ossicles are clearly visible grooves that give the ridge a distinctly striated appearance.

On either side of the ridge find the double row of bulbous ampullae of the tube feet of

the water vascular system. These protrude into the perivisceral coelom and are covered by its

peritoneum.

Body Wall

Examine the cut edge of a part of the body wall using moderate magnification of the

dissecting microscope. It consists of a thin, outer, ciliated epidermis, a thick, easily-seen

connective tissue dermis, and thin, inner peritoneum. Look at the dermis. It contains collagen

fibers and many calcareous dermal ossicles, or "little bones", which may have been crushed by

the scissors. Note that some of the ossicles bear spines.

Coelom

The echinoderm coelom has many subdivisions but only the perivisceral coelom and

water vascular system will be studied in this exercise. The perivisceral coelom is the largest of

the coelomic compartments and is the chief body cavity. Most of the space in the arms and disk

is perivisceral coelom (Figs 2, 3) and the viscera are located in it. The perivisceral coelom is

derived from the right and left metacoels of the larva (Fig 28-4).

Study the inner surface of the aboral wall of the arm which you removed earlier and set

aside. It is covered inside by a thin, transparent, ciliated epithelium, which is the peritoneum of

the perivisceral coelom. Activity of its cilia circulates coelomic fluid to distribute food and

oxygen to the surrounding tissues.

Figure 3 Cross section of an arm of Asterias.

Note the numerous small pores in the body wall and that the peritoneum extends into

them. These are openings into clusters of papulae. Using magnification and good light, look

straight into one of the pores and you will see that it opens into several papulae.

Many of the organs of the perivisceral coelom, such as the pyloric ceca and gonads, are

suspended from the body wall by mesenteries (Fig 3). The mesenteries were necessarily

destroyed when you removed the aboral body wall.

Digestive System

The short gut extends vertically from the mouth in the center of the oral disk to the anus

near the center of the aboral disk. It consists, in order, of mouth, esophagus, cardiac stomach,

pyloric stomach (with pyloric ceca), intestine (with intestinal ceca), and anus. It is lined

internally with a ciliated epithelium and is surrounded by the perivisceral coelom.

The mouth opens into a short indistinct esophagus which you will not see at present.

The esophagus opens into the large, thin-walled, orange cardiac stomach filling most of the

perivisceral coelom of the disk. When feeding, Asterias everts the cardiac stomach from the

mouth to surround its prey. Digestion begins extracellularly in the cardiac stomach while the

prey, and stomach, are still outside the mouth. Partially hydrolyzed materials are moved to the

pyloric stomach by ciliary currents. From here they enter the hollow pyloric ceca where both

extracellular and intracellular digestion take place. The products of digestion can be stored in the

cells of the pyloric ceca or, presumably, diffused into the surrounding perivisceral coelom.

Five pairs of so-called stomach retractor muscles anchor the cardiac stomach to the

sides of the ambulacral ridges (Fig 2). There is little muscle in them, however, and, as they are

primarily connective tissue, they probably are more important as anchors than as retractors.

The cardiac stomach opens at its aboral end into the much smaller pyloric stomach. The

pentagonal outline of the pyloric stomach makes it easy to recognize atop the cardiac stomach.

The ten large pyloric ceca are hollow diverticula from the pyloric stomach and the two

ceca of each arm share a common connection with the margin of the pyloric stomach. Each

cecum extends most of the length of its arm and consists of a long pyloric duct with numerous

branches (Fig 2). Tiny food particles are phagocytized by the cecal epithelium and digested

intracellularly.

Cut or tear one of the large branches of a pyloric cecum to show yourself that it is

hollow and has relatively thick walls. Its thick, endodermal epithelium is secretory and

absorptive.

The tiny, inconspicuous intestine extends aborally from the center of the pyloric stomach

to the anus. The lobed intestinal cecum is attached to the intestine. It, and the intestine, may

have been destroyed by the removal of the aboral disc.

Open the cardiac stomach and look inside. Push the billowy folds of the cardiac stomach

aside and trace the gut orally to the mouth. The short and indistinct region between the cardiac

stomach and the mouth is the esophagus.

Remove the gut from the animal to reveal the region around the mouth. This will

necessitate cutting the pyloric duct and the two stomach retractor muscles in each arm. Cut the

connection between the esophagus and peristomial membrane, taking note of the membrane as

you may not have seen it earlier.

Water Vascular System

Removal of the gut reveals most of the central features of the water vascular system.

Gently deflect the part of the aboral disk containing the madreporite and look below it for the

stone canal. This curved duct extends orally from the madreporite and has calcareous skeletal

rings for support. Because it is calcified, it is firm to the touch.

An obscure vertical partition, the interbrachial septum, is located between the bases of

each pair of adjacent arms. Thus, the star has five interbrachial septa. The stone canal is in the

interbrachial septum between arms I and II (Fig 4). Also in this septum is the soft axial complex,

which surrounds the stone canal but in gross dissection appears to be beside it.

Figure 4 Aboral view of the mouth frame of Asterias.

Trace the stone canal orally and note that it crosses a heavy, white, circular or pentagonal,

skeletal ring known as the mouth frame (Fig 4). The stone canal extends to the inner surface of

the mouth frame where it joins the inconspicuous (very) ring canal (= water ring) of the water

vascular system. The thin, membranous walls of the ring canal are not calcified and they adhere

closely to the inner curve of the mouth frame and cannot be distinguished from it. Its position,

however, is marked by nine small, soft, low, spongy Tiedemann's bodies on the inner margin of

the frame (Fig 4). These organs are evaginations of the ring canal. Typically 10 such bodies are

present, two associated with each interbrachial septum, but in Asterias one is missing where the

stone canal joins the ring canal so only nine are present. The lumina of these bodies are

continuous with the ring canal and it is thought that they remove foreign particles, by

phagocytosis, from the circulating fluid of the water vascular system.

The ring canal gives off five radial canals, one for each arm. These canals leave the

outside surface of the ring canal and pass along the ambulacral groove outside the ambulacral

ossicles of the skeleton (Fig 2) but they are difficult to see.

You have already seen the tube feet in the ambulacral grooves on the oral surfaces of the

arms. The aboral end of each tube foot narrows, penetrates the overlying ambulacral ossicle and

is continuous with an ampulla which bulges into the perivisceral coelom (Figs 2, 3). Like

everything else in the perivisceral coelom, the ampullae are covered by monociliated

peritoneum. Their lumina are also lined by peritoneum since the space inside is a coelomic

cavity, i.e. the water vascular system.

Use a microneedle to push an ampulla aside so you can see that it narrows orally and

becomes a slender tube penetrating the ambulacral ridge. Carefully insert a tiny needle into the

ampulla and pass it through the pore in the body wall to emerge on the other side in the middle of

a tube foot.

Reproductive System

Asteroids are gonochoric and fertilization is external. Each individual has a pair of

gonads in each arm. The gonads may be very large if the individual is sexually mature or, if the

specimen is immature or reproductively inactive, they may be so small as to be difficult to find.

If they are small, they will be located on the oral surface of the base of each side of each arm.

Every gonad connects to its own gonopore via an inconspicuous gonoduct. The tiny gonopores

are on each side of the base of the arm, on its aboral surface.

Excretory System

To date there has been no demonstration of a special osmoregulatory or excretory

systems in echinoderms. Coelomic and interstitial fluids are osmotically similar to seawater.

Leakage of water vascular system fluid across the pressurized tube feet is countered by the

slightly higher osmolarity of water vascular system fluid. The end product of nitrogen

metabolism is ammonia, which in asteroids is eliminated by diffusion from the papulae and tube

feet.

Respiratory System

In echinoderms the hemal system does not distribute oxygen to the tissues. Instead, most

major coelomic spaces are associated, at least indirectly, with respiratory surfaces and gasses are

transported by the circulating coelomic fluid. The papulae and tube feet are the respiratory

structures for the perivisceral coelom and the tube feet serve this purpose for the water vascular

system.

References

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Bullough WS. 1958. Practical Invertebrate Anatomy (2nd ed). MacMillan, London. 483p.

Chadwick HS. 1923. Asterias. Liverpool Mar. Biol. Comm. Mem. 25:1-63, pls. 1-9.

Chia F-S, Koss R. 1994. Asteroidea, pp169-245 in Harrison FW, Chia F-S. (eds). Microscopic anatomy of

invertebrates, vol 14. Echinodermata. Wiley-Liss, New York. 510pp.

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Medicine, US Public Health Service, Washington, DC. 605 pp.

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Questions

1. How would you characterize the skeletal system of most Echinoderms?

2. What is the function of the papulae in the echinoderm anatomy?

3. What is the function of the cardiac stomach?

4. What features do you expect to find on the oral surface of a sea star?

5. What is the function of the pyloric ceca?