CLAM 143 



through the heart. On either side of the stomach is a digestive 

 gland, or liver, which secretes certain digestive fluids that are 

 emptied into the stomach through two slender ducts. The intes- 

 tine, near the basal portion of the foot, is surrounded by the 

 gonads. 



Certain phases of the Clam's nutrition are unlike those studied 

 in other animals. In the first place, in securing food, the gills, 

 which ordinarily function solely for respiration, play a prominent 

 part. A constant current of water is brought into the mantle 

 cavity in which the gills are suspended. This water current is 

 due to the action of cilia on the gills. The current is drawn into 

 the mantle cavity through the ventral inhalent siphon, and, after 

 passing through the gills, leaves the animal through the dorsal 

 exhalent siphon. As the water passes through the gills small 

 living organisms and also very small particles of organic material, 

 which may be utilized for food, are strained out from the water. 

 The food materials are then carried by the action of special gill 

 cilia to the anterior end of the body where they come into contact 

 with the ciliated labial palps around the mouth. The cilia of the 

 latter beat in such a manner as to sweep the particles into the 

 mouth opening. 



The food thus taken into the mouth passes through the esopha- 

 gus, into the stomach, and then into the intestine, through the walls 

 of which the digested food materials are absorbed and passed to the 

 circulatory system. The indigestible materials pass on through 

 the intestine and are egested at the anal opening. They are 

 carried to the exterior through the exhalent siphon by the action 

 of the out-going water current. 



2. Vascular System 



The Clam has an efficient vascular system which, although some- 

 what different in details, consists of the same fundamental parts 

 noted in the animals previously studied. Certain features of the 

 vascular system recall those found in the Crayfish. The heart lies 

 in a membranous chamber, the pericardial cavity, and consists 

 of a large muscular ventricle through which the intestine runs. 

 Leading from the anterior end of the ventricle is the anterior 

 aorta, and from the other end, the posterior aorta. An en- 

 largement of the posterior aorta, soon after leaving the heart, is 

 the bulrus arteriosus, which acts as a reservoir for blood forced 



