The Class Asteroidea The class Asteroidea contains about 1,500 species of sea stars (commonly known as starfishes). Most starfish have a dorsoventrally flattened body. Starfish have a central disk to which five, or a multiple of five, sturdy arms are attached. Sea stars are common along rocky coasts where they eat clams, oysters, and other bivalves. The five-rayed body has an oral (mouth) and aboral (upper) side. Spines project from the endoskeletal plate through the thin dermis. Pincerlike pedicellarie keep the surface free from particles. Gas exchange is conducted by skin gills. On the oral surface, each arm has a groove lined with tube feet. A sea star feeds by everting its stomach. It positions itself over a bivalve and attaches tube feet to each side of the shell. By working tube feet in an alternating fashion, it opens the shell open. Only a small crack is needed to insert its cardiac stomach into the prey. Stomach enzymes begin digesting the bivalve as it is trying to close its shell.
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