262 ZOOLOGY 



and most of the surface of the gill, are lined with ciliated cells, 

 the cilia of which, beating more strongly toward the cloacal cavity, 

 cause a current of water to flow over the gills and through the 

 holes into the cloacal cavity. Cilia are also found lining the inner 

 surface of the mantle and on the labial palps, which we shall 

 take up later. 



Structure of Gills. — The internal structure of the gills is such 

 that blood slowly circulates through a network of thin-walled 

 spaces, which, in the margin of the gill, are separated from the 

 water by only a single layer of thin cells. Through this layer 

 oxygen is taken by osmosis from the water, and carbon dioxide 

 given up. The latter gas is passed off in the water through the 

 excurrent siphon. 



Food Getting. — The cilia of the gills (and of the mantle cavity 

 in general) play an important part in food getting. The clam, 

 because of its sedentary life, must receive its food in the water 

 which enters the mantle cavity. Food consists principally of one- 

 celled animals and plants. This food is collected by the cilia sur- 

 rounding the ostia, or holes in the gills, and is passed to the lahial 

 palps, four little flaps which surround the mouth. The mouth 

 may be found at the anterior end of the visceral mass. The ciliated 

 palps act as lips and pass the food on into the mouth. 



Food Tube. — The food tube and its digestive gland (the latter a greenish 

 mass easily seen through the body wall) occupies part of the visceral mass. 

 It is a thin-walled tube which makes several turns before leaving the body. 

 The stomach is a slight enlargement surrounded by the dark-colored diges- 

 tive gland. This gland has the same function as the pancreas of higher 

 animals. 



Circulation of Blood. — The circulation of the blood in the clam 

 is of chief interest to us because of the curious heart, which is 

 well developed, and has somewhat the same action as in the 

 higher animals. The heart may easily be found in a living clam 

 and the rate of beating counted. It is located near the surface 

 of the dorsal side of the body close to the hinge ligament. It 

 consists of two chambers, an auricle, which receives the blood, 

 and a ventricle, which by muscular contraction pumps the blood 

 on its course. The heart is surrounded by a thin-walled 



