212 AN INTRODUCTION TO THE STUDY OF FOSSILS 



Water, admitted through the branchial siphon into the 

 branchial chamber of the mantle cavity, is driven in a rapid 

 continuous current into the gills by the cilia bordering them. 

 From the gills the water passes in a strong current into the 

 cloacal chamber and thence through the exhalant siphon to the 

 exterior. The mantle likewise functions in respiration, being 

 a thin-walled reservoir for blood in contact with the water. 



Food as well as oxygen is brought to the animal in the water 

 introduced by the branchial siphon into the mantle cavity. 

 The principal food is diatoms, though according to the location 

 and the season the clam feeds also on small crustaceans and 

 rotifers, protozoons and larvae of mollusks. These small 

 food particles are swept in with the current and become en- 

 tangled in the mucus poured out by the gland cells of the gills. 

 Movements of the cilia on the gills urge these food masses on 

 to the mouth. This lies in the median line just below the an- 

 terior adductor muscle. Triangular flaps, the labial palps, 

 unite above and below it to form an upper and a lower lip. The 

 food is swept by the cilia on these palps into the mouth and 

 thence passes through the oesophagus into the stomach. Here 

 it is digested by the fluid poured in from the digestive gland 

 or '' liver," a paired dark brown mass surrounding the stomach. 

 The intestine leads from the stomach as a narrow tube which 

 makes several convolutions in the visceral mass, and passing 

 backw^ard, opens into the exhalant siphon just over the posterior 

 adductor muscle. 



The chief organ of the circulatory system is the heart, situated 

 saddle-like upon the intestine, dorsally near the hinge. It con- 

 sists of one ventricle and two auricles, and is inclosed in a 

 membranous chamber, the pericardium. (This is the only 

 body coelome possessed by the pelecypod.) The blood is a 

 colorless liquid and forms an important part of the mass of the 

 body. It is pumped by the ventricle into two aortae, the an- 

 terior and the posterior, which branch into arteries and open 

 sinuses, thus penetrating all parts of the body with its load of 



