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CnidariansJellyfish corals sea anemones and hydroids are members of the phylum Cnidarians are either solitary or form huge colonies and are carnivores that stingFlatworms and molluscs have literally stolen the cnidarian defensive strategy When they eat a cnidarian ingested cnidocytes arent fired instead theyre transported to the surface of the flatworm or molluscs and used for its own protectionCnidarians are diploblastic with a body wall formed from an outer epidermis ectoderm and inner gastrodermis endoderm There is a layer of mesoglea or mesenchyme between the layersOrganized at the metabolic wastes is accomplished by diffusion across the body wallAsexual reproduction which can occur in polyps and some medusa includes budding and fragmentation Cnidarians are dioecious gonads may form from the epidermis or gastrodermis and male and female medusae release their gametes into the surrounding water In some species the female broods the developing embryosInstead cnidarians have oral and aboral surfaces One of the most distinctive features of the Cnidaria is their radial symmetry and it gives us some clues to how they function Absence of directed movementNerve net distributed throughout the body as Nervous systemSensory structures are evenly spaced around the whole organism allowing it to react equally well in all directions Watch for the position of rhopalia that detect gravity these sensory structures are found around the bell margin of the jellyfish medusa They always contain a statocyst for balancegravity and sometimes ocelli for light detection tentacles and any other structures radially arranged The anemones and corals Anthozoa show biradial symmetry Even though most of their structures are radially arranged around the oralaboral axis there is at least one important paired structure that restricts the planes of symmetry to only two When you look at the anemone watch for the syphonoglyphs and the arrangement of the internalsepta to see why these animals are biradial rather than radialDiploblastictissuegradeorganization
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