%8 ANIMAL FORMS 
acts asa 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, 7) 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 
penetrating the ventricle of the heart. Traversing the in- 
testine 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, 2), 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 latter 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- 
