MOLLUSCA. 441 



strengthened by a shell. In these last, the shell in the 

 Limacina is a spiral univalve, covering the abdominal vis- 

 cera, and in the Hyalea, where it serves the same purpose, 

 it approaches in character a bivalve shell. It is, however, 

 destitute of a hinge, the two valves being united together 

 at their caudal margins, and there is no appearance of a 

 transverse adductor muscle. 



The organs of motion in all the genera consist of two 

 fins, or membranaceous expansions, one being seated on each 

 side of the head. They have no foot wherewith to crawl, nor 

 any suckers by which they can adhere to objects. They 

 are, therefore, free animals moving about in the water by 

 means of their fins, and probably possessing, at the 

 same time, a power of varying their specific gravity, as they 

 are capable of varying, to a certain extent, the form of their 

 bodies, and of enlarging or reducing their dimensions. 

 There is nothing peculiar in their nervous system. 



The organs of digestion differ greatly from those of the 

 cephalopoda, which we have already considered. They are 

 generally regarded as destitute, of eyes and ears. Their 

 tentacula are either seated on the head, forming two com- 

 plicated branches of filaments, or spread along the margin 

 of the tunic. There are no arms for seizing the food. The 

 mouth, however, is furnished with lips ; and, in some, an 

 appearance of a tongue at the entrance of the gullet. The 

 salivary glands are two in number, lengthened, descending 

 a considerable way into the abdomen, and pouring their 

 contents, by means of their excretory canals, into the cavi- 

 ty of the mouth. The gullet, after being encircled by the 

 nervous collar, suffers an enlargement, which has been 

 termed a Crop, contiguous to which is the stomach. Both 

 these cavities exhibit muscular ridges on the inner surface. 

 The liver surrounds the stomach, is intimately united 

 with its contents, and pours in its bile by numerous pores. 



