A FRESH-WATER CLAM 79 



Clams feed upon minute organisms and organic particles 

 contained in the water. Some of the water in the mantle cavity 

 is drawn by the ciliated oral palps into the mouth and passes 

 through the alimentary tract, where organic substances con- 

 tained in it are digested and absorbed. The clam usually lies 

 with its forward end buried in the sand and its hinder end with 

 the siphons projecting into the water. 



Exercise 9. Draw a diagrammatic view of the digestive system. 



The Reproductive System. The sexes are separate. The repro- 

 ductive glands (testes or ovaries) are very similar to each other 

 and consist of a pair of cream-colored masses which fill a large 

 part of the visceral mass. They communicate with the outside 

 through a pair of minute openings, one on each side of the visceral 

 mass near the base of the gills. The opening can often be located 

 by pressing out from them eggs or sperm. The eggs, as soon as 

 laid, pass into the water tubes of the outer gills of the mother, 

 where they hatch. The young larvae are very immature and are 

 called glochidia ; they leave the mother and attach themselves 

 to the sides of fishes by means of a pair of sharp projections on 

 the ventral edges of the valves of the shell, where they lead a 

 parasitic life. While here they undergo a metamorphosis and 

 finally attain the adult structure, when they detach themselves 

 and drop to the bottom. Look for glochidia in your specimen. 



The Nervous System. This consists of three pairs of ganglia : 

 the cerebral ganglia, or brain, the pedal ganglia, and the visceral 

 ganglia, and the nerves proceeding from them. Each of the last 

 two pairs is joined with the brain by a pair of nerve connectives. 



First find the visceral ganglia. They are a small grayish or 

 pinkish mass on the ventral surface of the posterior adductor 

 muscle with nerves radiating in all directions. Two of these 

 nerves, the cerebrovisceral connectives, will be seen passing for- 

 ward, one on each side of the visceral mass. 



Next find the brain. It consists of a pair of ganglia situated 

 above the mouth, just behind the anterior adductor muscle. The 

 two ganglia are not so close together as those of the visceral pair ; 

 they lie on either side of the muscle and are united by a com- 



