Pelecypoda. 113 



the water that lies outside of the gill through these openings 

 into the space within the gill. The water then passes up 

 through the open top of the gill into the cloacal chamber, 

 and back out of the excurrent siphon. The four gills are 

 so many narrow, V-shaped troughs with their sides full of 

 holes. Instead of filling at the top and leaking out at the 

 holes in the sides, these troughs are filled through the holes 



Ventricle 



Cross section through heart. Cross section through posterior adductor muscles. 

 Section at A . Section at B. 



FIG. 69. CLAM. 



Water currents through the gills. Compare with Figs. 68 and 71. 



in the sides, and overflow above, only the water cannot 

 run down the sides of the trough, but must pass back and 

 out through the upper siphon. (Figs. 68, 69, and 70.) 



The Food of the Clam, and how Obtained. The clam 

 lives on microscopic plants and animals and on minute par- 

 ticle^ oj[ w organic matter. This material is supplied by the 

 current*of water which is continually passing into the lower 



