222 ZOULOGY, 
specialized group, in the number of species and indi- 
viduals. 
CLASS I.—LAMELLIBRANCHIATA (Acephala, Bivalves). 
General Characters of Lamellibranchs.—This group is 
represented by the oyster, clam, mussel, quohog, scallop, 
etc. By a study of the common clam (Mya arenaria Linn.) 
one can obtain a fair idea of the anatomy of the entire class, 
as it is a homogeneous and well-circumscribed group. The ’ 
clam is entirely protected by a pair of solid limestone shells, - 
connected by a hinge, consisting of a large tooth (in most 
bivalves there are three teeth) and ligament (Fig. 155 C 1). 
The shells are equivalve, or with both valves alike, but not 
equilateral, one end (the anterior) being distinguishable from 
the other or posterior, the clam burrowing into the mud by 
the anterior end, that containing the mouth of the mollusk. 
The hinge is situated directly over the heart, and is there- 
fore dorsal or hemai. On the interior of the shells are the 
two round muscular :mpressions made by the two adductor 
muscles and the pallial impression, parallel to the edge of 
the shell, made by the thickened edge of the mantle. On 
carefully opening the shell, by dividing the two adductor 
muscles, and laying the animal on one side in a dissecting 
trough filled with water, and removing the upper valve, the 
mantle or body-walls will be disclosed; the edge is much 
thickened, while within, the mantle where it covers the el- 
liptical rounded body is very thin. The so-called black 
head, or siphon, is divided by a partition into two tubes, the 
upper, or that on the hinge or dorsal side, being excurrent, 
the lower and larger being incurrent—a current of sea-water 
laden with minute forms of life passing into it. Each orifice 
is surrounded with a circle of short tentacles. This siphon 
is a tubular prolongation ofthe mantle-edge, and is very ex- 
tensible, as seen in Fig. 155, A; it is extended, when the 
clam is undisturbed, from near the bottom of its hole to the 
level of the sea-bottom. In the fresh-water mussel (Unio, 
Fig. 156) the two siphonal openings are above the level of 
