Other Molluscs
About 800 species ofchitons(class Poly-placophora ) are known, all marine. They can be readily identified by the eight over-lapping shell plates that cover their slightly arched dorsal surface (Fig. 7.26). Their in-ternal organs are not coiled as in snails. Almost all chitons are restricted torocky shores. Most species use the radulato graze algae from the rocks. Many of them return to a homesite after feeding.One species, however, captures smallcrustaceans and other invertebrates with aflap-like extension of the mantle that sur-rounds the mouth. The 350 or so species of tusk shells,or scaphopods,(class
Scaphopoda ), havean elongate shell, open at both ends andtapered like an elephant tusk. They live insandy or muddy bottoms. The narrow endof the shell protrudes from the bottom