MOLLUSC A. 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. 



CIRCULATING 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 lungs 

 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 



