CLASS OF THE PTEEOPODA. 475 



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

 nished 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. 

 454) 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 pointof the tentacles. 

 Finally, the nervous system is less developed than in the pre- 

 ceding class, and is com posed 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 waters, but most are marine. In general they 

 are formed to creep, as the slug, the lymneae (Fig. 143), the 

 porcelaine (Fig. 158), &c. ; but sometimes they are intended 

 only to swim, as for example the carinaria (Fig. 457). 



CLASS OF THE PTEEOPODA. 



J610. The pteropoda are small molluscs having a distinct 

 , formed for floating and swimming by means of two fins 

 placed like wings on either side of the neck (Fig. 361). 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, and 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 

 side a large fold, covering all the other part of the animal 

 (Fig. 458), and sometimes even is so united to its fellow of 

 the other side, as to leave openings only behind and before, 

 and to form two long tubes for the water necessary for respi- 

 ration. A shell, composed of two valves, covers this mantle in 



