140 MOLLUSCA. 



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 cavity 

 of the mouth. The gullet, after being encircled by the ner- 

 vous 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. The in- 

 testine is short, and, after making one or two turns, ascends 

 and terminates in the neck near the mouth. 



The circulating system in this class has been but very 

 imperfectly investigated. The pulmonic vessels are un- 

 known, but systemic veins, a single auricle, ventricle, and 

 aorta, have been detected. The heart, in some, is situate 

 on the left, in others, on the right, side of the body. 



The aerating organs exhibit very remarkable differences. 

 In the Clio they are in the form of a fine net- work on the 

 surface of the fins ; in the Pneumodermon they are conjec- 

 tured to form leafrlike ridges on the caudal extremity of the 

 body ; or if these ridges are to be considered as particular 

 kinds of fins, the gills may be sought for on the membrana- 

 ceous expansions of the neck. In the Hyalea the branchiae 

 form a complex band on each side of thejbody, at the lateral 

 opening of the shell. 



The animals of this class are all hermaphrodites. There 

 is a common cavity, a vesicle, penis, vas deferens, and tes- 

 ticle, together with an oviduct and ovarium. These open 

 near the mouth on its ventral margin. There is nothing 

 known with respect to the appearance of the eggs, the pe- 

 riod of propagating, or the form of their young. 



