CLASS OF THE GASTEBOPODA. 471 



(Fig. 8), the nautilus (Fig. 450), &c. With these are arranged 

 the ammonite shells, which have some analogy with those of 

 the nautilus, but are found only in a fossil state. 



Fig. 452. The Ammonite. 

 CLASS OF THE GASTEBOPODA. 



606. The gasteropoda are molluscs which have a head, 

 and move by means of a fleshy disc or foot placed under the 

 belly (Fig. 453), or of a fin formed by the same part of the 

 body (Fig. 457) : this class, which has as its type the snail, 

 is very numerous, and is composed chiefly of animals lodged 

 in a single shell, having generally the form of a cone, or 

 rolled into a spiral ; some species are, on the contrary, en- 

 tirely naked, an the slug. The body is elongated, and ter- 

 minated anteriorly by a head more or less developed, in which 



Fig. 453. Casque. 



is the mouth, provided with fleshy tentacles, varying in num- 

 ber irom two to six ; the back is covered with a mantle pro- 

 longed more or less backwards, under the form of a mem- 

 branous sac, and which secretes the shell ; finally, the belly 



