MOLLUSKS 73 



occur along our seacoasts or are plentifully distributed in 

 the fresh-water streams and lakes. > They are distinguished 

 from other mollusks by a greatly compressed body, which 

 is enclosed in a shell consisting of two pieces or valves 

 locked together by a hinge along the dorsal surface. Eais- 

 ing one of these valves, the main part of the body may be 

 seen to occupy almost completely the upper (dorsal) part 

 of the shell (Fig. 45), and to be continued below into the 

 muscular hatchet-shaped foot (/.), which aids the clam in 

 plowing its way through the sand or mud in which it lives. 

 Arising on each side of the back of the animal and extend- 

 ing its entire length is a great fold of skin, which com- 

 pletely lines the inner surface of the corresponding valve 

 of the shell. These are the two mantle lobes (m) instru- 

 mental in the formation of the shell, and enclosing between 

 them a space containing the foot and a number of other 

 important structures, the most conspicuous of which are 

 the gills (</), consisting of two broad, thin plates attached 

 along the sides of the animal and hanging freely into the 

 space (mantle cavity) between the mantle and the foot. 

 Owing to this lamella-like character of the branchia or gills 

 the class derives its name, lamellibranch. To illustrate the 

 relations of these various organs to one another the clam has 

 been compared to a book, in which the shells are repre- 

 sented by the cover, the fly-leaves by the mantle lobes, the 

 first two and last two pages by the gills, and the remaining 

 leaves by the foot. In the clams, however, the halves of 

 the mantle, like the halves of the shell, are curved, and 

 thus enclose a space, the mantle cavity, which is partly 

 filled by the gills and foot. 



Unlike the other mollusks which usually lead active 

 and more aggressive lives, the clams show scarcely a sign of 

 a head and tentacles, and other sense organs are likewise 

 absent from this region. The mouth also lacks definite 

 organs of mastication, and as devices for catching and 

 holding food are not developed, the food is brought to the 



