THE SHELL-ANIMALS 



407 



in some species may be many inches long so as to enable the clam 

 to live some distance down in its burrow. The currents are due to 

 numerous cilia which cover organs within the shell (see Oyster, 338). 

 Organs within the Shell. (D or L) The fleshy parts of a clam, 

 which may be seen after removing one valve of the shell, are as fol- 

 lows : 



(1) The two great adductor muscles which hold the valves to- 

 gether. One is near the anterior, the other near the posterior end. 



(2) Mantle lining 



each valve, and be- 



tween the two folds 

 of mantle is the body 

 of the clam. 



(3) The foot is a 

 firm muscular struc- 

 ture, which may be 

 projected out from 

 between the folds of 

 the mantle at the an- 

 terior-ventral edge. 

 It is large in river 

 clams, and small in 

 the long-necked 

 clams, which do not 

 move much. 



(4) Between the 



foot and mantle on either side are two plate-like gills. Water that 

 enters at the siphon flows around these and through pores into 

 cavities in the gills, and thence through the gill-chamber above the 

 gills and out by exhalent current. Blood (colorless) flowing in blood- 

 capillaries in the walls of the gills is able to exchange oxygen and 

 carbon dioxide with the surrounding water. In short, the gills are 

 the organs of respiration. 



(5) Above the foot at the anterior end of the animal is the 

 mouth, just below the anterior adductor muscle. There are no jaws, 

 for the food consists of small organic particles brought by currents of 

 water. 



(6) On each side of the mouth are two triangular flaps (called 

 labial palps). These serve to direct the currents of water so as to 

 carry particles of food to the mouth. 



(7) From the mouth the alimentary canal (stomach and intestine) 

 extends down into the foot, where it is much coiled, then it runs up 



FIG. 142. Organs of a clam exposed by removal of 

 left valve of the shell. A , anterior ; P, posterior ; 

 D, dorsal ; V, ventral ; a, mantle ; g, gills ; m, 

 muscles that close the valves together ; /, foot ; 

 o, position of mouth ; u, umbo of shell ; I, labial 

 palp. Arrows show direction of ingoing and out- 

 going currents of water. (From Brooks.) 



