The Central Nervous System of Three 
Bivalves 
WILLIAM A. HILTON 
Lima Deliscens. 
The central neryous 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, while 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 the ganglia, the cells are large or ganglionic and small or ordinary nerve 
cells. 
Sunset Clam, Psammobia californica 
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 yentral 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. 
