22 Our Food Mollusks 



to its opening into the stomach. As represented in the 

 figure, the latter appears as a simple dilation of the di- 

 gestive tube. Surrounding it on all sides, are the digestive 

 glands, which pour their secretion into it through short 

 but wide ducts. The digestive glands constitute what 

 is commonly called the liver in anatomical descriptions of 

 many invertebrate animals; but it is not similar to the 

 liver of vertebrates, either in structure or function. Its 

 secretion has the power of rendering fluid and changing 

 chemically the digestible parts of the food. The gland 

 is always of a dark color, that varies somewhat in dif- 

 ferent bivalves, and every one has noticed it in the rup- 

 tured bodies of oysters and clams. 



The intestine arises from the posterior end of the 

 stomach. Its course is downward and backward, and 

 in the lower part of the body it bends in a way character- 

 istically different in different bivalves, before finally as- 

 cending to the region in front of the heart. Coursing 

 straight backward on the dorsal side of the body, it passes 

 directly through the heart in most bivalves, and then 

 over the posterior adductor muscle where it ends, the anal 

 opening of the tube being so situated that the strong 

 current of water leaving the body immediately carries 

 away the fecal matter. The parts of the digestive tract 

 in other bivalves have much the same arrangement. 



The Vascular System. It rarely happens that the 

 blood of invertebrate animals is colored, though there 

 are one or two exceptions to it even in the bivalve group. 

 In our edible mollusks, it is a nearly colorless fluid, cir- 

 culating through the body along very definite paths. As 

 in all other cases, it carries liquid food obtained from the 

 walls of the digestive tract, and oxygen received in the 

 gills and mantle, to all the living tissues of the body. At 



