CLAMS, MUSSELS, AND OYSTERS. 39 



buried at some depth below the level of the sand or mud in 

 which it occurs, it requires this extension of the openings 

 to reach the sea-water above. 



38. It may be stated here, that the current of water 

 passing into the general cavity of the shell not only carries 

 the particle of food upon which the animal subsists, but con- 

 veys the pure sea-water to the gills by which it breathes, 

 the gills performing the same function for animals living 

 immersed in water as the lungs perform for creatures which 

 breathe air. All bivalves depend upon currents of water to 

 convey their food to them. 



While, in the snails, the creatures could go in quest of 

 food, having the power of protruding the head from the 

 shell, and their mouths furnished with means to bite or rasp 

 their food, in the bivalves there is really no head, they hav- 

 ing only a little opening directly under the anterior adductor 

 muscle, which is the mouth, and into which the particles of 

 food are swept. 



39. If, now, the clam is opened, the edges of the mantle 

 will be found much thickened and united, except a small slit 

 near the front edge, through which can be protruded a small, 

 tongue-shaped foot. Powerful muscles will be found at the 

 base of the united siphons or tubes, which move the siphons 

 in and out, and an examination of the inside of the shell will 

 show where these muscles are attached. The pallial line, 

 instead of running directly from the anterior adductor im- 

 pression to the posterior one, is abruptly curved back, and 

 forms a sharp bend, as it turns again to the posterior ad- 



