408 APPLIED BIOLOGY 



and along the dorsal side of the animal to its end near the exhalent 

 part of the siphon. A digestive gland (greenish in color) lies near 

 the anterior part of the alimentary canal. 



(8) A heart and a pair of kidneys lie in the body of the animal 

 dorsal to the gills. Blood-vessels extend from the heart through all 

 the tissues of the body. 



(9) Ganglia (masses of nerve-cells) lie near the mouth, in the 

 foot, and near the posterior end ; and these are connected by nerve- 

 cords. Small nerves extend to various organs of the body. 



(10) The reproductive glands (ovaries or spermaries) lie above the 

 foot in the body-cavity. Their cells escape into the cavities (gill- 

 chambers) above the gills. The sperm-cells are carried out by the 

 exhalent current of water and then carried by inhalent currents into 

 other individuals (females) with egg-cells ready for fertilization. 



In the common fresh-water clams the fertilized eggs fall into 

 cavities in the gills and remain until developed into bivalved larvae. 

 Such clams collected in late summer or fall may have the outer 

 gills greatly thickened by eggs or young larvae (sometimes three 

 million in one clam). They finally escape in the exhalent current. 

 These larvae have sharp teeth on edges of their shells, and by these 

 they attach to skin or gills of fishes. Having attached, the skin of 

 the fish grows over (encysts) the clam larva, and so it remains while 

 it begins to metamorphose into the clam form. Finally, after 

 several weeks, the larva drops from the skin of the fish, falls to 

 bottom of the stream, pushes the lower edge of its shell into the mud, 

 and begins its life as a clam. 



Problem : Can you think of any way by which clam larvae which 

 are unable to swim might be distributed far upstream in rivers? 

 River clams probably originated near the sea. How have they been 

 distributed even to the very sources of some of the smallest brooks of 

 great river-systems like that of the Mississippi ? 



The larvae of marine clams have no such parasitic development in 

 fish, but escape from the gill-chamber as a ciliated larva able to 

 swim freely for some time before it metamorphoses into a clam. 

 Hence, marine clams may be widely distributed by swimming of 

 their larvae ; also by tides and ocean currents. 



338. Relatives of the Clams: Lamellibranchs. The terms 

 " clam " and " mussel " are applied to a number of marine and 

 fresh-water species of mollusks which have a bivalved shell. 

 There are two common types of river clams, one with thick 

 shells and one with thin shells. Of each type there are 



