78 ANIMAL FORMS. 



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. 



77. 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. 45, B, I) 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. 45, /*), 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 capil- 

 laries and veins, but in spaces or sinuses among the mus- 

 cles and various organs, constituting a somewhat indefinite 

 system 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- 



