Central Nervous System of Mytilus 
Californianus 
WILLIAM A. HILTON 
The cerebral ganglia are well separated from each other, but the smallest of the 
three groups of ganglia in the nervous system. A large cephalic branch goes to the 
palps, smaller lateral ones supply adjacent parts in the mouth region. No attempt 
was made at this time to follow peripheral branches very far. 
The single mass of the pedal ganglion may be easily seen to be composed of a 
right and a left half. The pedal connections with the main trunk are somewhat 
smaller than the long connectives and hardly larger than some of the other branches 
of the ganglion, notably the large lateral caudal branches. The caudal branches 
of the pedal are chiefly three on each side, the more lateral being very large and the 
medial the smallest. They penetrate and supply the foot and viscera. 
The visceral ganglia are more widely separated than the cerebral and much 
larger. The large caudal branches pass over the posetrior adductor muscles to be- 
come supplied to the muscle and to the mantle. The smaller lateral branch runs 
out laterally dividing soon into two to supply the gills. 
In the cerebral ganglion the fibers form a broad connection across the middle 
line. There are a few cells along the course of the commissure. Nerve cells are 
found inclosing the central fibrous mass. The cells are three or four layers thick except 
at certain places where there are none. The cells are of several sorts: First—those 
that stain deeply with hematoxylin. Some of these may be neuroglia cells, they are 
rather small cells and some seem to have very little cell body; second—large cells 
with clear protoplasm with distinct fibrillar structure. The processes of these are 
long, in some cases may be followed for some distance; third—there are some very 
small cells that do not stain deeply. These may some of them be neuroglia cells, 
others may be nerve cells in some special physiological condition. 
In the fibrous areas of the ganglia, larger and smaller strands are evident, with 
only a few cells in central portions. 
The cell areas about the cerebral ganglion differs slightly at various points, 
but there is no marked massing into groups. 
The pedal ganglion, like the cerebral, has a central fibrous core covered with a 
rather even mass of cells of large and small size, but the cephalic and caudal regions 
have the thicker masses of cells. The ventral sides of the pedal has more cells than 
the dorsal. Two sides of the ganglion are well marked from each other, although 
broadly connected by fibers. 
The large visceral ganglia are more complicated in structure than the others, 
but a similar general arrangement of cells is found. 
The peripheral distribution of nerves was not followed at this time. For the 
general anatomy of bivalves one of the most recent works gives a detailed account 
of peripheral distribution in a bivalve: 
Splittstosser, P., 1913. 
Zur Morphologie des Nervensystems von Anodonta cellensis Schrot. Zeit. f. wiss. 
zool. Bd. CIV 3 heft. 
(Contribution from the Zoological Laboratory of Pomona College.) 
