The Central Nervous System of 
Nucula and Malletia 
WILLIAM A. HILTON 
These bivalve forms are grouped among the simplest of the molloscs. It is espe- 
cially from the condition in Nucula as described by Pelseneer '91, that the conception 
of the most anterior ganglion being composed of four ganglia, has its chief support. 
Drew °01, who has also studied Nucula, believes that the lobes of the ganglion in 
Nucula are superficial and that the four connectives coming from the ganglion may 
be interpreted in another way. That is, that one pair of nerves may represent an 
otocystic branch partly fused with the connective. This view seemed reasonable to 
him as Stempel ’99 in Solenyma found the otocystic nerves arose directly from the 
cerebral ganglion. 
The two species of this group used for study were collected at Laguna Beach. 
Nucula castrensis Hinds, occurs abundantly at low tide under rocks. It is rather 
small for dissection, but very good complete series were obtained and stained in 
hematoxylin. Malletia faba Dall, was much less abundant. Specimens were obtained 
from holdfasts or from dredging. Although this was a larger species, gross dissection 
was not very easily carried out on any of the specimens, but good series were made. 
The ganglia of Nucula are easily studied in section. The cerebral mass seems 
composed of one main mass, partly divided into four subdivisions, the two central 
most completely fused, and the lateral quite distinct in places. The central portion 
might represent the cerebral ganglia and the lateral, the pleural if we take that inter- 
pretation. The pedal ganglion is made of right and left parts quite completely fused 
except at the margins. The pedal mass is the smallest of the three chief ganglionic 
areas. The visceral ganglia are quite widely separated and a little larger than the 
pedal mass. 
The ganglia of Malletia are in general plan similar to those of Nucula, the great- 
est differences being in the cerebral mass. The cerebro-pleural mass seems almost one. 
In most sections it is very compact and a little more complicated in structure than 
the ganglion of Nucu/a. However there are two small ventral ganglionic branches 
or small ganglia attached to the ventral side of the cerebral mass. These small 
ganglia may represent the visceral. Farther back in a cross section series as the 
cerebral mass disappears two other small branches take origin and run parallel to 
the nerves from the ganglionic cords. These two branches on each side seem to run 
together before the pedal ganglia are reached. Neither of these pairs of nerves seems 
connected with an otocyst. 
At the cephalic end of the cerebro-pleural ganglion the large ganglionic cords 
are in evidence. A little distance from the cephalic end on the dorsal side there 
are quite large groups of cells down from the surface and surrounded by nerve fibers. 
The course of the fibers here is quite complex. On the ventral lateral sides of the 
ganglia are paired light areas of fibers which may be traced into the fibers of the 
ganglionic cords. 
The pedal ganglion is small and much as in Nucula. The visceral ganglia are 
larger and widely separated. 
In both Nucula and Malletia young specimens were used for study. In Nucula 
