160 ANIMAL ACTIVITIES. 
Nutrition. In the clam the various processes con- 
cerned in nutrition are more distinctly separated than 
in any of the other sub-kingdoms below the Arthrop- 
oda. When the food enters the mouth it passes down 
the throat to the stomach. The dark portion of the 
body surrounding this alimentary canal is the liver, 
which secretes digestive fluids and pours them on the 
food. The intestine beyond the stomach passes directly 
Fic. 125.—Digestive Tube of Fresh-water Mussel (Anodon’. m, mouth; 
Z, liver; s, stomach; z and 7, intestine; «, anus; #, pericardium; 
R, kidney; s.c., chamber above gills; g, gullet. 
through the heart and opens into the cloacal chamber 
in the current of water which passes out of the upper 
siphon. 
The digested food is taken up by the blood and 
carried to all parts of the body by the circulating 
system. The most important organ of this circulating 
system is the heart, which is situated just below the 
hinge. It has a ventricle and two auricles. 
In its course, part of the blood is purified by the gills 
