116 PELECYPODA. 
Crass PELECYPODA. 
(Lamellibranchiata, Conchifera, Bivalves.) 
Acephalous mollusks, or those without a head, are laterally 
symmetrical, the organs being enclosed in a mantle, one 
leaf or fold of which envelops each side, and is itself covered 
and protected by a valve of the bivalve shell—which is always 
present. They all breathe by means of gills only, and are there- 
fore inhabitants of water, either salt or fresh. The mantle is 
usually open on the edge, but sometimes its leaves are united 
more or less, when the mantle is said to be closed; it is some- 
times tubularly prolonged posteriorly into a siphon. Between 
the lobes of the mantle lay the gills or branchiz, two on each 
side, leat-like and striate. The most prominent organs of the 
body are the liver and viscera. The cerebral ganglion is situated 
above the mouth, and communicates with the other near or dis- 
tantly situated ganglia. The mouth is at one extremity, the 
anus at the other; the former is provided with four small, trian- 
gular, fleshy leaflets, the extremities of the lips, used partly as 
tentacles. The heart is in the dorsal region; it has but one 
ventricle, and the circulation is simple. The foot is a somewhat 
fleshy mass, atrophied in the adherent species, usually suitable for 
digging a, but rarely very useful for other locomotion. The prin- 
cipal muscles are: (1) those controlling the movements of the 
foot, which have their insertion upon the valve, partly near the 
hinge, partly near the adductors, forming pedal scars; (2) the 
adductor muscles, which, running through the mass of the 
animal, are inserted upon the middle or sides of each valve, 
forming adductor scars or impressions. When the adductor 
muscle is single, it is generally centrally situated, as in the 
oyster, and such bivalves are termed monomyary; when double, 
one is at either side of the valve, and such mollusks are termed 
dimyary. The contraction of these muscles closes the valves ; 
when relaxed, the valves open by reason of an elastic ligament 
which joins them together at the dorsal or hinge-line. 
Mostly dicecious. No sexual union, fertilization being accom- 
plished by the surrounding water containing the male element. 
Shell composed of two valves, but with occasionally smaller, 
supernumerary pieces about the hinge; this latter is either a 
plain line, or more or less thickened internally, and provided 
with interlocking teeth and fossets. Some shells, from their 
shape, cannot be completely closed, or at least portions, front or 
back, or both, are always gaping 
The form of the shell, number and position of the retractors, 
sear of the mantle-margin, the hinge, its condition as to teeth, 
