Radiata - Two phyla (the "RADIATA") exhibit...

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1 Cnidaria : hydroids, jellyfish, sea anemones and corals Two phyla (the “RADIATA”) exhibit radial symmetry throughout life* *Both are considered the most primitive eumetazoans (multicellular animals with true tissues ) Ctenophora : comb jellies The major lineages of evolution in animals Fundamental evolutionary divergence among Eumetazoa (multicellular with true tissues) , but based on what? true tissues cellular only Gastrulation proceeds differently in animal embryos: Fig. 3.5 In the RADIATA, gastrulation ends here with only 2 germ layers (a diploblastic embryo with a “2-way”, blind” or “incomplete” gut (only 1 opening) Gut: gastrocoel, archenteron Phylum Cnidaria bearing stingers ( cnidae )
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2 ~ 10,000 species in the phylum - most are marine - a few freshwater species exist (e.g., the common lab animal, Hydra ) There is no collective common name for cnidarians (jellyfish, corals, sea anemones, and hydroids are common names for the different classes) Cnidarians are either solitary or form colonies (below) : “brain” coral Portuguese man-of-war - Cnidarians derive their name from cnidocyte cells characteristic of the phylum. - Cnidocytes contain organelles called cnidae (cnidum, sing.); the most common of these are nematocysts that eject “harpoon-like” stinging or adhesive threads. - deliver either a toxin or an adhesive - With the exception of a few herbivorous coral species, most cnidarians are carnivorous : they entangle and poison prey using nematocysts prevalent in cells in the epidermis & mostly on the tentacles. Nematocysts (= a “stinging” cnidum) • a cellular organelle capped by an operculum within a cnidocyte (cell) • a capsule with coiled, inverted thread with spines, toxin, adhesive • tactile stimulation causes explosive thread eversion “deployed” nematocysts each cnidocyte has a “hair-trigger”, the cnidocil , which is usually activated by physical contact causing the cell to burst open and extrude its contents physical or chemical contact changes permeability of the nematocyst; water rushes in; change in osmotic pressure pops operculum open and the cnidum (nematocyst) is forced out inside each cnidum (nematocyst) is a coiled, barbed thread which explodes out of the cell cnidocyte is destroyed & replaced Fig. 7.4
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3 Several kinds of cnidae grouped into three functional categories: volvents glutinants have a sticky secretion that sticks to the prey (also used penetrants have barbs at their tips and penetrate the prey, often injecting a toxin causing paralysis Functions: prey capture defense adhesion movement aggression! Although cnidae are unique to the phylum
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Radiata - Two phyla (the "RADIATA") exhibit...

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