258 COMPARATIVE ZOOLOGY. 
beak, but it is not perforated. It is the point from which 
the growth of the valve commences. Both Brachiopods 
and Lamellibranchs are headless; but in the latter, the 
mouth points the same way as the umbo, 
2. é., toward 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. 
Fic. 215.—Pearl Oyster The valves are united to the animal by 
(Meleagrina margariti- ° 
Yera); one-fourth nat- One muscle (as in the Oyster), or two 
ural size. Ceylon. (as in the Clam), and to each other by 
a hinge. In some species, as the IFresh-water Mussel, 
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 between 
the edges of the valves, so that it is 
squeezed as they shut, like the spring in 
a watch-case. Such bivalves are said to 
be edentulous. But in the majority, as 
the Clam, the valves also articulate by 2 
interlocking parts called teeth. The Fre. 216.—salt-water 
valves are, therefore, opened by the ea aes 
ligaments, and closed by the muscles.  °*** 
The margin of the shell on which the ligament and teeth 
are situated is termed the hinge-line. 
Lamellibranchs breathe by four plate-like gills (whence 
the name), two on each side underneath the mantle (Fig. 
78). In the higher forms, the mantle is rolled up into 
two tubes, or siphons, for the inhalation and exhalation of 
water. They feed on infusorial particles filtered from the 
water. A few are fixed, the Oyster, ¢.¢., habitually lying 
on its left valve, and the Salt-water Mussel hanging to the 
