MOLLUSKS 95 



acts as a lid. As locomotion is out of the question, the foot 

 never develops, and the shell is held by only one adductor 

 muscle, whose point of attachment in the oyster is indicated 

 by a brown scar in the interior of the shell. 



88. Internal organization. It is thus seen that the ex- 

 ternal features of the clam are variously modified, according 

 to the life of the animal, but the internal organization is 

 much more uniform. In nearly every species the food con- 

 sists of floating organisms, which are driven by the palps 

 into the mouth and on to the simple stomach, where it is 

 subjected to the solvent action of the fluids from the liver 

 (Fig. 51, B, 1} before entering the intestine. This latter 

 structure is usually of considerable length, and in the active 

 species extends down into the foot, and it is also peculiar in 

 passing through the ventricle of the heart. Traversing the 

 intestine the nutritive portion of the food is absorbed, and is 

 conveyed over the body by a circulatory system more highly 

 developed than in the higher worms. On the dorsal side 

 of the clam, in a spacious pericardial chamber, the large 

 heart is situated (Fig. 51, /^), consisting of a median highly 

 muscular ventricle surrounding the intestine and of two 

 thin auricles, one on either side. From the former, two 

 arteries with their numerous branches convey the blood to 

 all parts of the body, where it accumulates, not in capilla- 

 ries and veins, but in spaces or sinuses among the muscles 

 and various organs, constituting a somewhat indefinite sys- 

 tem of channels which lead to the gills and kidneys. In 

 these organs the blood delivers up the waste which it has 

 accumulated on its journey, and absorbing a supply of 

 oxygen, it flows into the great auricles, which in turn pass 

 it into the ventricle to circulate once more throughout 

 the body. 



The excretory apparatus, consisting usually of two kid- 

 neys, of which one may degenerate in many snails, bears a 

 close resemblance to that of the annelids. In the clam, for 

 instance, each consists of a bent tube symmetrically ar- 



