THE CLAM. 139 



touch, and the opening is always protected by small, 

 finger-like or fringe-like processes which fold over the 

 opening and protect the clam from the introduction of 

 grains of sand and other injurious substances. The 

 upper siphon is not so closed, but is usually quite 

 open, and the finger-like processes diverge from 

 the edges. 



The water comes through the lower siphon into 

 the gill chamber. Here it bathes the gills. Each gill 

 consists of a double layer of vertical rods fastened 

 together by transverse fibers, and the whole surface of 

 the gill is covered by a layer of ciliated cells. The 

 gill of the clam is one of the easiest objects in which 

 to observe ciliary motion. It is this ciliary motion 

 which is the principal cause of the current of water 

 into the lower siphon. 



Some, of the water that enters the gill chamber 

 passes between the vertical rods of each gill into the 

 space between the two layers. This water goes up- 

 ward between the two layers of the gill, into the cloacal 

 chamber. The water which does not enter the gill 

 passes forward over the palpi and in front of the 

 inouth, and upward into the cloacal chamber, 

 where it meets the water that has come through the 

 gills, and with it passes backward and out through the 

 upper, or excurrent, siphon. 



In consequence of the very short siphons, the 

 fresh-water clam usually buries itself in the sand only 

 the length of the shell ; but the salt-water clam, such 

 as comes into our markets, has siphons which may be 

 extended six or eight inches, thus permitting it to 

 sink much deeper into the sand and still keep its 

 siphons at the surface. 



The food consists of very small particles of animal 

 or vegetable matter which are carried into the gill 

 chamber by the current that enters the incurrent 

 siphon. It passes around in front of the mouth, 



