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Cnidarians & Ctenophorans copyright cmassengale 1 CNIDARIANS Phylum Cnidaria • About 9000 species • Most are marine (jellyfish, coral, sea anemones, Portuguese man-of war) • Hydras are found in freshwater • All have arm-like tentacles Cnidarian Characteristics • Radial symmetry • Hollow gut -Gastrovascular cavity has single opening (serves as both mouth and anus) • Two tissue layers (diploblastic) • Outer epidermis and inner gastrodermis Cnidarian Characteristics •Either tubular or bell shaped •Tentacles surround mouth •Only animal with cnidocytes (stinging cells) found in the tentacles •Nematocyst – coiled, harpoon-like stinger inside cnidocyte that shoots out Cnidarian Characteristics •Cnidocytes contain poison to paralyze prey •Tentacles bring food into mouth •Inner gastrodermis secretes digestive juices into gastrovascular cavity which digests food and circulates nutrients Extracellular Digestion Food digested outside of cells Cnidocyte Coiled nematocyst Nematocyst Barbed, threaded stinger capsule’s lid trigger nematocyst Cnidarian Systems •Have a simple nerve net & sensory receptors •Muscles help in directional movement & capturing prey mesoglea-filled bell tentacles Jellylike mesoglea between two cell layers supports cnidarian Two Main Body Plans of Cnidarians UPRIGHT TENTACLES TENTACLES HANG DOWN epidermis Polyp mouth mesoglea gastrodermis Medusa Mesoglea THICK Mesoglea THIN Cnidarian Diversity • Scyphozoans: Jellyfish • Anthozoans: Sea anemones Corals • Hydrozoans: Hydra • Cubozoa Box Jellies Cnidarian Diversity Coral polyps Sea Anemone Jellyfish Coral polyps Anthozoans Anthozoan Characteristics Sea Anemones Brightly colored & Resemble flowers Solitary polyps Feed on invertebrates & fish Corals Most are colonial Build limestone case Live as polyps in their case Sea Anemones & Coral • Exist only as Polyps • Clownfish live in anemone tentacles • Corals build a limestone case which build up & form reefs ANEMONE CORAL Distribution of Coral Reefs Hydrozoans Hydrozoans Hydra •Exist only as polyps •Reproduce by budding •Freshwater only •Form resistant zygotes when environment is bad •Move by somersaulting with tentacles Hydra Hydra Feeding Food in Gastrovascular Cavity Portuguese man-of-war • Colonial Hydrozoan (not a single organism • Tentacles sting prey such as fish & humans • Polyps in colony feed • Has gas-filled air float Hydrozoan Life Cycle Most hydrozoans alternate polyp and medusa forms, as in the life cycle of Obelia Obelia Life Cycle (Hydrozoan) Larva called a planula reproductive polyp male medusa female medusa ovum sperm zygote feeding polyp polyp forming planula Fig. 25-15a, p.411 Cubozoans Box JellyFish Scyphozoans Scyphozoan Characteristics Jellyfish • Medusa is dominant body form • Go through small polyp stage during life cycle •Stinging tentacles •May live close to shore or the open ocean Parts of a Jellyfish Some Jellyfish Show Luminescence Jellyfish Life Cycle CTENOPHORANS Ctenophoran Characteristics • Called comb jellies • All marine • Approximately 100 species • May be spherical, ovoid, or elongate in appearance • Bioluminescent Ctenophoran Characteristics • Radial symmetry • Have eight plates of fused cilia that resemble combs • Move by beating cilia • Most of the body is jelly-like mesoglea • Body & tentacles covered with sticky mucus to catch prey Comb Jelly Anatomy Beating Cilia Ctenophore: Comb Jelly Ctenophore 42 43
