The Central Nervous System of Three 

 Bivalves 



WIl.I.IAM A. IllI.TON 



Lima Deliscens. 



The central nervous system forms a rather compact mass of nervous tissue, with 

 certain special local thickenings where nerve cells are abundant. As in Pectin, as 

 described by Drew, the visceral ganglion is the largest, but it is not so widely sepa- 

 rated from the other ganglia as in Pectin. Neither is it so complicated in structure. 



There are, on each side, three main branches from the visceral ganglion, the most 

 caudal goes over the adductor muscle to the mantle. The next, the smallest main 

 branch, goes to the gills, Vfhile the last branch, the largest, is chiefly a mantle branch, 

 which divides after leaving the ganglion. 



The cerebro-pleural ganglia are connected medio-caudally by a looped com- 

 missure, the other large medial branch on each side runs to the rather large pedal 

 ganglion, while near it is the small otocystic branch, much as in Pectin. The large, 

 more cephalic branch runs towards the mouth region and gives off a number of 

 branches, about seven. 



The pedal ganglion is made up of two nearly distinct parts and from each of 

 these lateral parts a branch runs into the foot. 



The visceral ganglion is more complex than the others in structure, but there 

 are only a few distinct fiber tracts. 



In all tlie ganglia, the cells are large or ganglionic and small or ordinary nerve 

 cells. 



Sunset Clam, Psammohia (■alijitrnica 



The cerebral ganglia are of fair size and not widely separated. There is a 

 cephalic branch supplying the mouth region and palps and a more ventral branch 

 also on each side, supplies neighboring parts. The commissure between the two 

 ganglia is rather narrow considering the size of these centers. 



The Pedal ganglion is small and gives little indication of being divided into 

 two parts. The two connectives come to it and two rather large branches leave. 



The visceral ganglion is large and especially well developed. This is because 

 of the large siphons and their necessary abundant nerve supply. The siphons are 

 capable of being extended some distance from the shell. The ganglion is complexly 

 lobed on superficial view. There are on each half at least six little lobes which 

 represent to some degree groups of nerve cells. On each side in addition to the 

 large connective branch there are branches as follows: (1) a large branch to the 

 gills, (2) a large trunk which divides again into mantle branches. One of its branches 

 going to the dorsal siphon, (3) a small dorsal branch, (4) a small ventral branch, (5) 

 another large mantle branch which sends some strands to the ventral siphon, (6) 

 another large mantle branch, (7) a small branch to the posterior adductor muscle. 



