GENERAL ZOOLOGY 



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Fig. 13.10. A chiton, dorsal and ventral views. The see;mented skeletal plates are bounded 



laterallv by a peripheral fold of the mantle. In 5, the broad foot has been released from its 



attachment to the substratum; note the lateral row of e;ills, protected bv the overhanging 

 mantle fold. (Photographs by John F. Storr. ) 



some species concealed by an overlying integument. The ventral foot is 

 broad and flat, modified for creeping and for adhesion to stones and other 

 smooth surfaces (Fig. 13.10). The edge of the body, forming the mantle, 

 surrounds the foot dorsolaterally. In the angle between foot and mantle a 

 row of small gills extends into the mantle cavity on each side. An anterior 

 region, covered dorsally by the overhanging mantle and distinct from the foot, 

 constitutes the head and bears the mouth. The anus opens posteriorly into 

 the mantle cavity, between the paired nephridiopores and reproductive 

 openings. 



Chitons live in shallow water in rocky localities, creeping slowly or lying 

 with the edges of the body firmly fixed against the substratum. Their food 

 consists of algae which are rasped from the rocks by the action of a tongue- 

 like radula resembling that of a snail (p. 384). Internally, the digestive, 

 circulatory, excretory, reproductive, and nervous systems are generally similar 

 to those described for pelecvpods, but less well developed and of a relatively 

 primitive type. 



Amphineurans are believed to resemble more closely than any other exist- 

 ing type of moUusk the common ancestral form from which all have evolved. 



THE CLASS GASTROPODA 



The gastropods, represented by the snails and slugs, are mollusks in which 

 the head is well developed, the foot is typically broad and flattened, muscular, 

 and adapted for creeping, and the visceral mass is commonly coiled upward in 

 a spiral. The shell covers the visceral mass and usually exhibits a correspond- 



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