OYSTERS, CLAMS, MUSSELS 219 



Instead of a series of ventral ganglia and a more or less 

 specialized brain as is found in the insects, worms and some 

 other invertebrates, the nervous system of the mussel consists 

 of three scattered principal groups of ganglia connected by 

 nerve cords. Lying one on each side of the esophagus are the 

 cerebro-pleural ganglia. They are connected with each other 

 by a nerve which passes over the esophagus, and by larger 

 nerves with the pedal ganglion in the foot and the visceral 

 ganglion which lies near the posterior adductor muscle. 



The sexes of the mussels are separate, that is the ova and 

 spermatozoa are produced in different individuals, but the 

 reproductive glands, or gonads, of the two sexes are very similar. 

 They form a glandular mass of tissue filling the base of the foot. 

 The ducts from the ovaries and testes open near the base of the 

 gills. The spermatozoa are carried from the body with the 

 water that passes out through the exhalant siphon, and find 

 their way to another mussel with the incoming water currents. 

 The eggs pass into the supra-branchial chamber, but instead of 

 passing on out of the body remain there until they are fertilized 

 by the spermatozoa from another individual. They then 

 drop down into the outer gills, which serve as brood chambers. 

 Here the young are held for some time, and develop bivalve 

 shells which enclose them. In some species the margin of the 

 shell is provided with stout hooks, but in others it is without 

 them. Thus armed, the young, or glochidia, as they are called, 

 pass into the water. When touched the shell closes quickly 

 and firmly. If the young come in contact with the fins or gills 

 of a fish the snapping shut of the shell may serve to attach them 

 to it. The forms with hooks on their shells are more often 

 found on the fins of the fish, the bookless kind on the gills. 

 Once attached, their presence causes an irritation of the tissues 

 of their host which results in a growth or cyst that soon covers 

 them over. In this condition they remain for some time, 

 drawing their nourishment from the host and undergoing the 

 transformations that change them to small mussels which fi- 

 nally drop to the mud where they continue their growth, feed- 

 ing on small organisms, both plant and animal, which are 

 taken from the water entering th,e mouth cavity. The 



