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Multicellular and Tissue Levels of Organization
Chapter 9 Multicellular and Tissue Levels of Organization
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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 Origins of Multicellularity At least 800 million years—Precambrian Colonial hypothesis Syncytial hypothesis
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Figure 9.1 Evolutionary relationships of the Porifera, Cnidaria, and Ctenophora.
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Figure 9. 2 Two hypotheses regarding the origin of multicellularity
Figure 9.2 Two hypotheses regarding the origin of multicellularity. (a) Colonial hypothesis. (b) Syncytial hypothesis.
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Phylum Porifera Asymmetrical or superficially radially symmetrical
Three cell types: pinacocytes, mesenchyme cells, and choanocytes Central cavity, or a series of branching chambers, through which water circulates during filter feeding No tissues or organs
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Table 9.1
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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
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Figure 9.4 Morphology of a simple sponge.
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Figure 9.5 Sponge spicules.
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Figure 9.6 Water Currents and Body Forms.
Complex sponges have increased surface area for filtering large volumes of water.
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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)
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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
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Figure 9.7 Development of sponge larval stages.
Parenchymula larva. (b) Amphiblastula larva. (c) Gemmule. (d)
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Phylum Cnidaria Radial symmetry or modified as biradial symmetry
Diploblastic, tissue-level organization Gelatinous mesoglea between the epidermal and gastrodermal tissue layers Gastrovascular cavity Nerve cells organized into nerve net Specialized cells, called cnidocytes, used in defense, feeding, and attachment
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The Body Wall Epidermis Gastrodermis Mesoglea Outer cellular layer
Ectodermal origin Gastrodermis Inner cellular layer Endodermal origin Mesoglea Jellylike Cells present but origins are epidermal or endodermal
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Figure 9.8 Body wall of a cnidarian.
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Nematocysts Cnidocytes
Epidermal or gastrodermal cells that produce cnida 30 types Nematocysts used in food gathering and defense
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Figure 9.9 Cnidocyte structure and nematocyst discharge.
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Figure The generalized cnidarian life cycle involves alternation between a sexual medusa stage and an asexual polyp stage.
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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.
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Reproduction Medusa Polyp Dioecious External fertilization most common
Planula larva Polyp Budding produces miniature medusae.
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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
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Figure 9.11 Obelia structure and life cycle.
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Figure 9. 12 Gonionemus medusa
Figure Gonionemus medusa. The velum is unique to members of the Hydrozoa.
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Class Staurozoa Figure Members of the class Staurozoa are marine and lack a medusa stage. Lucernaria janetae is shown here.
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Class Scyphozoa Marine Medusa dominant in life history
Lacks velum Cnidocytes epidermal and gastrodermal Gametes gastrodermal Dioecious
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Figure 9.14 Representative scyphozoans (a) Mastigias qinquecirrha and (b) Aurelia labiata.
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Figure 9.15 Structure of the scyphozoan medusa of Aurelia.
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Figure 9.16 Aurelia life history.
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Class Cubozoa Cuboidal medusa Tentacles hang from corners Tropical
Dangerous nematocysts Figure The sea wasp, Chironex fleckeri.
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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
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Figure (a) The giant sea anemone (Anthopleura xanthogrammica) and (b) a sea anemone (Callictis parasitical) living in a mutualistic relationship with a hermit crab.
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Figure 9.19 The structure of the anemone, Metridium sp.
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Reproduction Asexual Sexual Pedal laceration
Longitudinal or transverse fission Sexual Monoecious or dioecious External fertilization produces planula. Monoecious species Protandry Male gametes mature first.
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Corals Stony Reef forming Lack siphonoglyphs
Cuplike calcium carbonate exoskeleton Asexual budding expands colony. Symbiotic relationship with zooxanthellae
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Figure 9.20 A stony coral polyp in its calcium carbonate skeleton.
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Corals Octacorallian Warm waters Eight pinnate tentacles
Eight mesenteries Internal protein or calcium carbonate skeleton
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Figure 9.21 Octacorallian corals (a) Ptilosaurus gurneyi and (b) Gorgonia ventalina.
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Phylum Ctenophora Diploblastic or possibly triploblasitic
Biradial symmetry Gelatinous, cellular mesoglea True muscle cells Gastrovascular cavity Nerve net Colloblasts Eight comb rows
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Table 9.3
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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.
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Figure 9. 22 (a) The bioluminescent ctenophoran Mnemiopsis sp
Figure (a) The bioluminescent ctenophoran Mnemiopsis sp. (b) The structure of Pleurobranchia. (c) Colloblast structure. (a)
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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
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Figure 9.23 Cladogram showing cnidarian taxonomy.
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