MOLLUSKS 79 



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. The 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 (p) 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- 



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