270 
COMPARATIVE ZOOLOGY. 
fluviatile or lacustrine, and a few are terrestrial air-breath¬ 
ers. All bivalves, and nearly all univalves, are aquatic. 
Each zone of depth in the sea has its particular species. 
Class I. —Lamellibranchiata. 
Latnellibranchs are all ordinary bivalves, as the Oyster 
and Clam. The shells differ from those of Brachiopods 
in being placed on the right tind left 
sides of the body, so that the hinge is on 
the back of the animal, and in being 
unequilateral and equivalved. 138 The 
umbo, or beak, is the point from which 
the growth of the valve commences. 
Fig. 224.—Pearl oyster Both Brachiopods and Lamellibranchs 
are headless; but in the latter the mouth 
urai size. Ceylon. p 0 i n ts the same way as the umbo, i. e ., 
towards the anterior part. The length of the shell is 
measured from its anterior to its posterior margin, and its 
breadth from the dorsal side, where the 
hinge is, to the opposite, or ventral, edge. 
The valves are united to the animal by 
one muscle (as in the Oyster), or two (as 
in the Clam), and to each other by a 
hinge. In some species, as some fresh¬ 
water Mussels, the hinge is simply an 
elastic ligament, passing on the outside 
from one valve to the other just behind 
the beak, so that it is on the stretch when 
the valves are closed, and another placed Fi«- 225. — salt -water 
between the edges of the valves, so that AtLm/c 
it is squeezed as they shut, like the spring coa8t8 ‘ 
in a watch-case. Such bivalves are said to be edentulous. 
But in the majority, as the Clam, the valves also articulate 
by interlocking parts called teeth. The valves are, there¬ 
fore, opened by the ligaments, and closed by the muscles. 
