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Brittle stars, central disk, long flexible arms, tube feet without suckers
sand dollars and sea urchins, unevenly spherical or disk-shaped, no arms, five rows of tube feet, Aristotle's lantern rings mouth
sea cucumbers, five rows of tube feet, modified tube feet as feeding tentacles, no spines, reduced skeleton
Why are there two invertebrate groups in phylum Chordata?
They exhibit a notochord, dorsal hollow nerve cord, pharyngeal gill slits, and a post anal tail. Intermediate characters.
lampreys, no jaw, larva resembles lancelet, cartilaginous skeleton, notochord is primary support, rudimentary vertebrae
sharks, skates, rays, jaws present, paired fins, mineralized skeleton, placoid scales
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