MOLLUSCS 113 
Bivalves. Bivalve Molluscs, as their name implies, have 
their softer parts enclosed in a shell consisting of two 
pieces—a right and a left valve, which in most forms are 
equal and similar. The valves are attached to each other 
along the back (a) by an elastic ligament; and (0) by 
muscles and connective tissue, which pass from the inner 
surface of one valve to the inner surface of the other. The 
elastic ligament may be outside the shell, in which case it 
extends from one valve to the other just behind the beak 
in such a way as to be tightly stretched when the shell is 
closed ; or it may be inside the shell placed between the 
hinge-lines of the two valves, perpendicular to the surface 
on which it rests. The closing of the valves is effected by 
the contraction of powerful muscles, known as adductor 
muscles which, as previously stated, pass from one surface 
to the other—whilst the opening is brought about by the 
elastic force of the ligament. When the animal wishes to 
open the shell, it has only to relax the adductor muscles, 
whereupon the elastic force of the ligament is sufficient to 
force the valves apart. 
The union of the valves is still further strengthened in 
most shells by means of a series of parts called teeth, 
which interlock with one another (see fig. 75), 
The animal lives on the small particles of nutrient 
matter brought to the mouth by ingoing currents of water, 
which pass over the gills and through the digestive tract. 
The currents of water may find their way to the mouth 
cavity direct, or may enter and leave through a siphon 
tube, there being one for the ingoing and another for the 
outgoing currents, The mantle lobes, a pair of delicate 
membranes, which hang down from the sides of the body, 
are attached to the lower inner surface of the shell, 
All these organs leave well-marked impressions or scars 
which are plainly visible when the animal is removed from 
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