MOLLUSKS 18 
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. Rais- 
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 (/t.), 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 (g), 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 
