MOLLUSKB 



403 



Shells. 



ried on by either one or more hearts. When there is only one, it 

 is situated so as to receive the blood from the gills, and circulate 

 it through the body. When there are two, the second is situated 

 so as to circulate through the gills the blood coming from the body. 

 In some species, there are three hearts ; and in this case, as there 

 are two sets of gills, a distinct heart is devoted to each. The blood 

 in the Mollusca is thin, of a bluish white, and always cold. 



The organs of digestion vary very much. Sometimes there 

 are organs for mastication, and sometimes not. Some species have 

 only a single stomach, and others have several ; the structure of 

 this organ, in some species^ very much resembling that of the 

 gizzard of birds. In some species there are four stomachs, which 

 bear a great analogy to those of the ruminating animals, and have 

 been supposed to answer a similar purpose. In the intestines there 

 is as great a variety. 



Mollusks are divided into the Cephalous, (those which have 

 heads), and the Acephalous, (or those which are destitute of 

 heads). The first are subdivided into the Cephalopods, naked and 



