412! PHILOSOPHY OF ZOOLOGY. 



side of the body ; in others, it is terminal, while, in a few, 

 it opens on the back. 



The digestive system is thus more simple in its structure 

 than in the higher classes It possesses neither pancreas, 

 spleen, nor mesentery. The calls of hunger are often at 

 distant intervals, and the power of abstinence is great. 



The characters furnished by the digestive system are 

 extensively 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 at- 

 tentively considered, as indicating the habits of the spe- 

 cies. 



Circulating System. The process by which the food is 

 converted into chyle, has not been satisfactorily traced, nor 

 has the existence of lacteals for the absorption of the chyle 

 been demonstrated. In this class of animals, the veins 

 seem to perform the offices both of lacteals and lymphatics. 

 The blood is white, or rather of a bluish colour. Its me- 

 chanical and chemical constitution yet remains to be inves- 

 tigated. 



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 appear 

 to be co-ordinate with any particular plan of external con- 

 figuration and manner, as we see in the case of the ptero- 

 poda and gasteropoda. In these, the organs of circulation 

 are very much alike, while the external forms exhibit very 

 obvious differences. 



The molluscous animals which respire by means of lungs 



