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Cnidarians CNIDARIANS Phylum Cnidaria • Cnidaria means “stinging cells” • About 9000 species • Most are marine (jellyfish, coral, sea anemones, Portuguese man-of war) • Hydras are found in freshwater • All have arm-like tentacles copyright cmassengale 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 polyp (tubular) or medusa (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 Two Main Body Plans of Cnidarians UPRIGHT TENTACLES TENTACLES HANG DOWN Motile - swim Usually sessile epidermis Polyp mouth mesoglea gastrodermis Medusa Mesoglea THICK Mesoglea THIN mesoglea-filled bell tentacles Jellylike mesoglea between two cell layers supports cnidarian 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 • Undigested food is released through the mouth – only one opening Extracellular Digestion • Food digested outside of cells • Digestion is completed in the gastrodermal cells (intracellular) Cnidocyte Coiled nematocyst Nematocyst Barbed, threaded stinger capsule’s lid trigger nematocyst Cnidarian Systems • Have a simple nerve net & sensory receptors (no brain) • Eyespots detect light • Muscles help in directional movement & capturing prey Cnidarian Systems • Respiration, Circulation and Excretion – all carried out by diffusion Cnidarian Systems • Reproduction • Asexual – polyps bud • Sexual – most cnidarians are separate sexes • Fertilization is external – take place in water Cnidarian Taxonomy • Kingdom Animalia • Phylum Cnidaria • 3 Classes: • Scyphozoa • Hydrozoa • Anthozoa Cnidarian Taxonomy • Scyphozoans “cup animals”: Jellyfish • Anthozoans “flower animals”: Sea anemones Corals • Hydrozoans “water animals”: Hydra 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 https://youtu.be/Pu_ijC8HFRU Some Jellyfish Show Luminescence Jellyfish Life Cycle 40
