3454chapter13 - Chapter 13, Cnidaria and Ctenophora Phylum...

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Chapter 13, Cnidaria and Ctenophora Phylum Cnidaria—knide – nettle, aria – pl. of like; like a nettle Cnidocytes—stinging cells after which the group is named 9,000-10,000 species, marine or freshwater Radial symmetry with oral and aboral ends Especially useful for sessile animals Tissue level of organization with some simple organs Diploblastic—ectoderm and endoderm (some have an ectoderm-derived mesoderm) Develop into epidermis and gastrodermis from these germ layers Mesoglea—matrix between these tissues Non-living, having a few cells, or with connec. tiss. + muscle
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Gastrovascular cavity—central cavity with single opening (mouth) Digestion (extracellular, in cavity) and respiration Cavity extends in radial and ring canals in larger cnidarians and may have many septa Tentacles—extensions on the oral end for food capture Nerve cells—simplest animals with true nerve cells, arranged in a “nerve net” -- again, “simple” refers to the level of complexity, not the evolutionary status. .. Structure and function Polyp (hydroid) and medusa (jellyfish) morphological types Some species are polyps, some are dimorphic, some are medusae
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contraction Colonial species may have polymorphism in its members Different selective forces acting on polyp and medusa forms of same species Cnidocytes—stinging cells Nematocysts—stinging organelle within the cnidocyte, chitinous barbs Cnidocil—trigger on outside of cnidocyte Operculum—cap over nematocyst Triggering cnidocil allows water to rush into capsule (140 atm. osmotic pressure)
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3454chapter13 - Chapter 13, Cnidaria and Ctenophora Phylum...

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