Chapter 9

Multicellular and Tissue Levels of Organization

Evolutionary Perspective• Porifera

– No tissues– Division of labor among independent cells– Independent origin from common animal ancestor– Choanoflagellate protists (?)

• Cnidaria and Ctenophora– Tissue level organization– Independent origins from common animal ancestor– Choanoflagellate protists (?)

• Origins of Multicellularity– At least 800 million years—Precambrian – Colonial hypothesis– Syncytial hypothesis

Figure 9.1 Evolutionary relationships of the Porifera, Cnidaria, and Ctenophora.

Figure 9.2 Two hypotheses regarding the origin of multicellularity. (a) Colonial hypothesis. (b) Syncytial hypothesis.

Phylum Porifera1. Asymmetrical or superficially radially

symmetrical2. Three cell types: pinacocytes,

mesenchyme cells, and choanocytes3. Central cavity, or a series of branching

chambers, through which water circulates during filter feeding

4. No tissues or organs

Table 9.1

Cell Types, Body Wall, and Skeletons• Pinacocytes

– Outer surface– Some contractile, others may be specialized into porocytes

• Mesohyl– Jellylike middle layer

• Mesenchyme cells– Amoeboid cells– Reproduction, secreting skeletal elements, transporting and storing food,

form contractile rings• Choanocytes

– Flagellated– Collarlike ring of microvilli– Water currents for filter feeding

• Skeleton– Spicules– Spongin

Figure 9.4 Morphology of a simple sponge.

Figure 9.5 Sponge spicules.

Figure 9.6 Water Currents and Body Forms.

• Complex sponges have increased surface area for filtering large volumes of water.

Maintenance functions• Filter feeding

– Bacteria, algae, protists, suspended organic matter– Trapped in choanocyte collar and incorporated into

food vacuole– Digestion by lysosomal enzymes and pH changes

• Nitrogeneous waste removal and gas exchange– Diffusion

• Coordination– Responses of individual cells (some coordination)

Reproduction• Monoecious• Choanocytes (and sometimes ameboid

cells) lose collars and flagella and undergo meiosis.

• External fertilization and planktonic larvae in most

• Asexual reproduction– Gemmules– Freshwater and some marine

(d)

Figure 9.7 Development of sponge larval stages. (a)Parenchymula larva. (b) Amphiblastula larva. (c) Gemmule.

Phylum Cnidaria1. Radial symmetry or modified as biradial

symmetry2. Diploblastic, tissue-level organization3. Gelatinous mesoglea between the

epidermal and gastrodermal tissue layers4. Gastrovascular cavity5. Nerve cells organized into nerve net6. Specialized cells, called cnidocytes, used

in defense, feeding, and attachment

The Body Wall• Epidermis– Outer cellular layer– Ectodermal origin

• Gastrodermis– Inner cellular layer– Endodermal origin

• Mesoglea– Jellylike – Cells present but origins are epidermal or

endodermal

Figure 9.8 Body wall of a cnidarian.

Nematocysts• Cnidocytes– Epidermal or gastrodermal cells that

produce cnida– 30 types• Nematocysts used in food gathering and

defense

Figure 9.9 Cnidocyte structure and nematocyst discharge.

Figure 9.10 The generalized cnidarian life cycle involves alternation between a sexual medusa stage and an asexual polyp stage.

Maintenance Functions• Gastrovascular cavity– Digestion– Gas exchange– Excretion– Reproduction– Hydrostatic skeleton

• Support and movement• Epitheliomuscular cells act against water-filled

cavity.• Nerve net coordinates body movements.

Reproduction• Medusa– Dioecious– External fertilization most common– Planula larva

• Polyp– Budding produces miniature medusae.

Class Hydrozoa• Mostly marine• Some freshwater• Unique features– Nematocysts only epidermal– Gametes epidermal and released to

outside of body–Mesoglea largely acellular–Medusae with velum

Figure 9.11 Obelia structure and life cycle.

Figure 9.12 Gonionemus medusa. The velum is unique to members of the Hydrozoa.

Class StaurozoaFigure 9.13 Members of the class Staurozoa are marine and lack a medusa stage. Lucernaria janetae is shown here.

Class Scyphozoa• Marine• Medusa dominant in life history– Lacks velum

• Cnidocytes epidermal and gastrodermal

• Gametes gastrodermal– Dioecious

Figure 9.14 Representative scyphozoans (a) Mastigias qinquecirrha and (b) Aurelia labiata.

(a) (b)

Figure 9.15 Structure of the scyphozoan medusa of Aurelia.

Figure 9.16 Aurelia life history.

Class Cubozoa• Cuboidal medusa• Tentacles hang

from corners• Tropical• Dangerous

nematocysts

Figure 9.17 The sea wasp, Chironex fleckeri.

Class Anthozoa• Colonial or solitary• Lack medusa• Cnidocytes lack cnidocils• Anemones and corals• Mouth leads to pharynx• Mesenteries divide gastrovascular cavity

and are armed with nematocysts.• Mesoglea with ameboid mesenchyme

cells

Figure 9.18 (a) The giant sea anemone (Anthopleura xanthogrammica) and (b) a sea anemone (Callictis parasitical)living in a mutualistic relationship with a hermit crab.

Figure 9.19 The structure of the anemone, Metridium sp.

Reproduction• Asexual– Pedal laceration– Longitudinal or transverse fission

• Sexual– Monoecious or dioecious– External fertilization produces planula.– Monoecious species

• Protandry– Male gametes mature first.

Corals• Stony– Reef forming– Lack siphonoglyphs– Cuplike calcium carbonate exoskeleton– Asexual budding expands colony.– Symbiotic relationship with

zooxanthellae

Figure 9.20 A stony coral polyp in its calcium carbonate skeleton.

Corals• Octacorallian–Warm waters– Eight pinnate tentacles– Eight mesenteries– Internal protein or calcium carbonate

skeleton

Figure 9.21 Octacorallian corals (a) Ptilosaurus gurneyi and (b) Gorgonia ventalina.

Phylum Ctenophora1. Diploblastic or possibly triploblasitic2. Biradial symmetry3. Gelatinous, cellular mesoglea4. True muscle cells5. Gastrovascular cavity6. Nerve net7. Colloblasts8. Eight comb rows

Table 9.3

Phylum Ctenophora• Cellular mesoglea and true muscle cells

suggest that members may be triploblastic.

• Locomotion by bands of cilia are called comb rows.

• Tentacles contain adhesive cells called colloblasts that capture prey.

• Monoecious with gastrodermal gonads– External fertilization leads to flattened larval

stage.

(a)

Figure 9.22 (a) The bioluminescent ctenophoran Mnemiopsis sp. (b) The structure of Pleurobranchia. (c) Colloblast structure.

Further Phylogenetic Considerations• Porifera

– Oldest fossil deposits– Choanoflagellate ancestors– Increases surface-to-volume ratio in syconoid and leuconoid

body forms evolved in response to selection for increased size.

• Cnidaria– Radially symmetrical ancestor

• Minority view suggest bilateral ancestor.– Molecular data and morphology suggests relationships

shown in figure 9.23.• Ctenophora

– Relationships to other groups uncertain but probably distant

Figure 9.23 Cladogram showing cnidarian taxonomy.