MOLLUSCA 



231 



rcLtL 



bod.CCLZ/ 



JO.W 



oeji 



lapping plates, forming a single anteroposterior series. This 

 enables them to roll up into a ball and thus protect the soft 

 parts of the body, which lie on the ventral surface. Projecting 

 beyond the edges of the row of plates on all sides is the mantle, 

 and on the ventral side of it lie the gills, sometimes forming a 

 complete row around the body, sometimes confined to the pos- 

 terior region. The 

 foot on the median 

 ventral surface of 

 the body is some- 

 times narrow, but 

 usually presents a 

 broad, flat surface, 

 extending from the 

 mouth to the anus, 

 and s e r v e s for 

 creeping. In the 

 mouth is a tongue, 

 or odontophore, 

 whose surface is 

 covered with sharp 



chitinous teeth ; a similar structure is found in many other 

 Mollusca and is called technically the radula or lingual ribbon 

 (Fig. 233). Organs of special sense are for the most part 

 entirely wanting or very rudimentary. The Chitons are chiefly 

 confined to the shores of the ocean, and the different species 

 vary greatly in size. 



CLASS II. GASTROPODA 



The Gastropoda (Gr. yaaTijp, stomach, and Trovq, foot) com- 

 prise the snails, slugs, limpets, and many others less familiar. 

 The foot is generally long, with a broad, flat sole for Creeping. 

 There is usually a head more or less sharply marked off from 

 the rest of the body. It bears one or two pairs of tentacles and 

 sometimes a proboscis with the mouth at its tip ; there is usually 

 a pair of eyes on the head, sometimes on the tips or sides of a 

 pair of tentacles, sometimes sessile, on the surface of the head. 

 The mouth is provided with a radula or lingual ribbon borne on 

 the odontophore. 



cu-T-t. j~a.<£.sezc 



FIG. 233. Triton nodiferus ; diagrammatic longitudinal ver- 

 tical section of buccal or mouth cavity, bod.cav, body-cavity; 

 cart, cartilage of odontophore; jaw, right jaw; oes, oesopha- 

 gus; rad, radula; rad.sac, radula sac. (After Parker and 

 Haswell.) 



