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 
ofthe 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 aérating 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 leaf-like 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 branchiz 
form a complex band on each side of the body, 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. 
