CLASS. OF THE PTEROPODA. 519 



608. The mouth of the gasteropodes is surrounded by con- 

 tractile lips, and sometimes armed with horny teeth occupying 

 the palate. In several other animals of this class the anterior 

 part of the gullet is very fleshy, and may be carried outwards 

 so as to form a proboscis. Sometimes the stomach is also 

 furnished with cartilaginous or osseous instruments adapted 

 to divide the food ; the intestine is turned on itself, and is 

 lodged between the liver and the ovary ; finally, the anus 

 (a, Fig. 540) is almost always situated on the right side of 

 the body, and is often found close to the head. 



609. In this class the organs of sensibility are less 

 developed than in the cephalopodes ; the tentacles which most 

 gasteropodes carry on their forehead serve only for touch, 

 and perhaps for smell. Their auditory organs consist only in 

 a pair of small membranous vesicles, and the eyes, which are 

 sometimes wanting, are very small, and of a very simple 

 structure ; they are sometimes adherent to the head, some- 

 times carried on the base, the side, or the point of the 

 tentacles. Finally, the nervous system is less developed than 

 in the preceding class, and is composed principally of a 

 cephalic ganglion, and of a thoracic ganglion re-united like a 

 collar around the gullet. Amongst these animals, some are 

 terrestrial, others live in fresh water, but most are marine. 

 In general they are formed to creep, as the slug, the limneus 

 (Fig. 166), the cowry (Fig. 181), &c. ; but sometimes 

 they are intended only to swim, as for example the carinaria 

 (Fig. 543). 



CLASS OF THE PTEROPODA. 



610. The pteropoda are small molluscs having a dis- 

 tinct head, formed for floating and swimming by means of 

 two fins placed like wings on either side of the neck (Fig. 

 184). Some are naked, and others have a shell, but their 

 history is not of sufficient interest to induce us to dwell 

 longer on it. 



CLASS OF THE ACEPHALA. 



611. The molluscs which we have hitherto been consi- 

 dering have all a distinct head; those which remain to be 

 spoken of are without it, arid show a greater simplicity in 

 their whole organization. Their body is entirely enveloped 

 by the mantle, like a book in its cover ; the skin of the back, 

 in fact, is adherent only in the middle, and forms on each 



