MOLLUSKS "9 
ranged on each side of the body (Fig. 45, B, &), and the inner 
ends (a), corresponding to the ciliated funnel of the anne- 
lid kidney, open into the pericardial cavity. Their walls 
are continually active in extracting wastes from the blood 
supplied to them, and these, together with the substances 
swept out from the pericardial cavity, traverse the tube and 
are carried to the exterior. In other mollusks the kidney 
may be more compact, or greatly elongated, or otherwise 
peculiar, but in reality they bear a close resemblance to 
those of the clam. 
78. Nervous system.—The nervous system, like the ex- 
cretory, differs considerably in different mollusks, yet the 
resemblances are fairly close throughout. In the clam the 
cerebral ganglia corresponding to the “brain” in annelids 
is located at either side, or above the mouth, and from it 
several nerves arise, the larger passing downward to two 
pedal ganglia (») embedded in the foot and to the visceral 
ganglia (v) far back in the body (Fig. 45, B). These nerve 
centers continually send out impulses which regulate the 
various activities of the body and also receive impressions 
from without. These come chiefly through the sense of 
touch, for in the clams the other senses are usually either 
feebly developed or altogether absent. 
79. Development.—In the mollusca new individuals al- 
ways arise from eggs, which are commonly deposited in the 
water and there undergo development. In the fresh-water 
clams the reproductive organ is usually situated in the foot 
(Fig. 45), while in the oyster and similar inactive species it is 
attached to the large adductor muscle. In these latter, and 
in many other marine forms, the eggs are shed directly into 
the sea, where they are left to undergo their development 
buffeted by winds and waves and subject to the attack of 
numerous enemies. Under such circumstances the chances 
of survival are slight, and for this reason eggs are laid in 
vast numbers, which have been variously estimated for the, 
oyster, for example, from two to forty million. Develop- 
