Although often ignored by collectors, probably because of their small size and 

 uniform appearance, they constitute an important item in the diet of fishes 

 and other aquatic animals. 



North America is extremely rich in varieties and numbers of fresh-water 

 mussels or clams. Many genera have apparently originated in the ideal en- 

 vironment of the Mississippi valley and have since spread widely. The shells 

 of the various species show great variation in size, shape and coloring, and 

 have attracted the interest of amateur and specialist since the settlement of 

 the country. 



The living clam is well protected by its shell, and not much of the animal's 

 body is usually visible. As one rows slowly along in shallow water, he may 

 see the undisturbed clam at its normal activities. The shell is usually half 

 buried in mud or gravel. The hinge joint is uppermost and the anterior end 

 tilted downward. Between the slightly separated valves of the shell, behind 

 the horny hinge, extend two tubular projections. The upper one is formed by 

 a smooth, mottled membrane and is called the exhalent siphon. Below and 

 behind it is another slightly larger opening, surrounded by a circle of mem- 

 branous tentacles. This is the inhalent siphon, which serves both for conducting 

 aerated water to the gill chamber and for taking in the minute organisms or 

 microscopic plants upon which the animal feeds. The clam usually takes up 

 a position with the siphons headed up stream, so that its food is brought down 

 with the current. The sensitive tentacles of the lower siphon warn the clam 

 to close if anything undesirable touches them. To the naturalist who is used to 

 marine animals the fringed inhalent siphon recalls the sea anemone. 



If one picks up the clam quickly, he may see that a large muscular part of 

 the body, called the foot, was protruding on the lower side of the animal from 

 between the valves of the shell. Since this foot has somewhat the shape of an 

 axe blade, clams are frequently referred to by zoologists as Pelecypoda, meaning 

 hatchet-footed. The foot serves to anchor the animal in a good feeding ground 

 or to enable it to crawl from place to place. As the clam progresses, this foot 

 and the edges of the shell plough a furrow in the soft mud, so that one can 

 easily trace the creature's wanderings. Most clams prefer water from one to 

 three feet deep, and move as conditions change. 



Within the shell and hanging down on each side of the body of the clam 

 are the gills, two on each side, the inner pair being the longer. Each gill re- 

 sembles a curtain folded upon itself or a double curtain continuous at the 

 bottom. The gills are crosswise divided into compartments called water tubes. 

 Some of the water tubes of the female clam serve as ovisacs during the breeding 

 season and may be discolored or distended with developing eggs. The ovisacs 

 make up the portion of the gills known as the marsupium. Clam taxonomy is 

 based largely upon the size and structure of the marsupium. 



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