246 ^"f ANIMAL KINGDOM 



I. Amphineura. A small group with a shell, if any, located dor- 

 sally and made of many spicules or of a longitudinal series of 

 plates. Includes the chitons. 



II. Gastropoda. A large group with a single dorsal shell, if pres- 

 ent, that is usually spiral in shape. Includes snails, slugs, whelks 

 and abalones. 



III. Pelecypoda. A large group with a pair of lateral shells, hinged 

 dorsally. Includes the bivalves, such as clams, oysters and scal- 

 lops. 



IV. Scaphopoda. A small group with a conical shell open at both 

 ends. These are called the tooth shells. 



V. Cephalopoda. A large group in which part of the foot forms 

 arms or tentacles surrounding the mouth. Includes the squids 

 and octopuses. 



105. Class Amphineura 



The chitons, which are common on the west coast of the United 

 States, are the most primitive class in the phylum, and illustrate the gen- 

 eralized moUuscan plan. They are found only in the oceans, where they 

 creep slowly over the rocks. Many live between high and low tide lines. 

 Some species remain in one place all the time, where they gradually 

 wear a depression in the rock. These feed upon the debris that settles 

 into their hole. Often, after the hole has become deep, encrusting 

 growths may obstruct the opening to such an extent that the chiton 

 can no longer get out. 



Chitons creep upon a broad foot, moving by a succession of small 

 contraction waves that pass forward from the posterior end. The broad 

 surface with its slimy secretions enables chitons to cling tenaciously. 

 The mantle extends out over the foot on all sides, enclosing a circular 

 mantle cavity below. Dorsally the mantle secretes a shell made of eight 

 segments. Because of the segmental structure of the shell a chiton is 

 able to roll into a ball, shielding the vulnerable ventral surface, if it is 

 torn loose from the bottom (Fig. 14.2). 



In the anterior part of the mantle cavity is the head, no more than 

 a tubular extension of the body bearing a mouth at its end. Well de- 

 veloped sense organs such as eyes or tentacles are lacking. 



In the floor of the mouth cavity lies the radula, with which the 

 chiton scrapes up its food. The radula is a thin flexible strip of toothed 

 skin that can be pulled around the end of a stiff tongue. In a typical 

 scraping movement (Fig. 14.3), the tongue is pushed out of the mouth 

 with the radula on its anterior and lower surface. The radula is then 

 pulled around the end of the tongue onto the upper surface, scraping 

 whatever the mouth is pressed against. Finally the tongue is withdrawn 

 and the debris on the radula is swallowed. 



The mouth leads to a long coiled intestine that ends posteriorly at 

 a short rectum and anus opening into the posterior part of the mantle 

 cavity. Anteriorly the intestine receives ducts from a pair of digestive 

 glands, presumed to secrete digestive enzymes. 



