MOLLUSCA. 17 
hunger are often at distant intervals, and the power of ab- 
stinence very great. 
The characters furnished by the digestive system are ex- 
tensively used in the inferior divisions of molluscous ani- 
mals. The form of the lips, the position of the mouth and 
anus, and the structure of the stomach, deserve to be atten~ 
tively considered, as indicating the habits of the species. 
CrrcuLATING SysteM.—The process by which the food 
is converted into chyme, has not been satisfactorily traced, 
nor has the existence of the lacteals for the absorption of 
the chyme been demonstrated. In this class of animals the 
veins seem to perform the offices both of lacteals and lym- 
phatics. The blood is white, or rather of a bluish colour. 
Its mechanical and chemical constitution yet remains to be 
investigated. 
The circulating system of molluscous animals, exhibits 
very remarkable differences in the different classes. In all of 
them, however, there is a systemic ventricle; but the other 
parts of the heart are not of constant occurrence. 
The circulating system furnishes few characters which 
can be employed in systematical arrangements. The struc- 
ture of the systemic and pulmonary vessels does not ap- 
pear to be co-ordinate with any particular plan of external 
configuration and manner, as we see in the case of the 
pteropoda and gasteropoda. In these, the organs of circu- 
lation are very much alike, while the external forms exhibit 
very obvious differences. 
The molluscous animals which respire by means of duags 
are few in number, and form a very natural tribe, which 
Cuvier has termed gasterpodes pulmones. In them the res- 
piratory organ is simple, consisting of a single cavity, on the 
