NUTRITION PROPER. 



339 



through the gills and out by the other tube of the siphon, 

 carrying with it refuse or excretions of all kinds. The 

 most remarkable thing about the digestive apparatus is 

 the enormous liver (nearly the whole of the dark part of 



$ 



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Fig. 223. — Vertical longitudinal section of Anodonta : pm, am, posterior- 

 anterior adductor muscles ; m, mouth ; st, stomach ; cce, caecum ; //, 

 heart ; f, foot ; G, gills ; ma t mantle. 



an oyster or a clam is liver), through which the long and 

 convoluted intestine winds, receiving in its course the 

 biliary secretion from many openings, and then, strange 

 to say, passing through the heart on its way backward 

 to the vent at the si- 

 phon. The stomach 

 and the winding course 

 of the intestine is shown 

 in Fig. 223. 



Gastropoda.— For 

 an example of these 

 take a snail. Gastro- 

 pods are much more 

 highly organized than 

 the acephala. They 

 have a distinct head 



and their food-taking is voluntary. The mouth is armed 

 with transversely ridged chitinous plates, which have 



Fig. 224. — Section through snout of a car- 

 nivorous gastropod showing the radu- 

 la, r, in place : /c, lingual cartilage ; 

 mph, muscle of pharynx ; m, mouth ; 

 ce, oesophagus. (After Lang.) 



