The Nervous System of Caecum Califor- 
nicum 
WILLIAM A. HILTON 
Specimens of this little gastropod mollosc from 2 to 3 mm. in length were the 
material for the study. Specimens were fixed and sectioned whole and a few good 
series were obtained. 
It seems rather remarkable that so small a species should have such a high organ- 
ization of the nervous system. The ganglia are large in proportion to the size of the 
animal and well developed. In all cases the exact limits of the nerves and connectives 
were not determined, but the chief ganglia were easily found. 
Quite well towards the head end a pair of buccal ganglia were found, these were 
small, widely separated and possessed only a few nerve cells. At about this level in 
cross sections the eyes make their appearance, one on each side. ‘They are simple, 
quite large and well provided with pigment. Below the level of the eyes and the 
bueeal ganglia, on the dorsal side of the esophagus, the much larger cerebral ganglia 
make their appearance. These probably are connected with the eyes but the connec- 
tions were not clearly seen in the sections. ‘The cerebral ganglia are closely united 
along the middle line. They occupy more than one half the diameter of the entire 
animal. The more caudal ends of these ganglia separate and run down, a little lateral 
to the esophagus. 
Below the esophagus and a little below the chief level of the cerebral ganglia, 
a region of more ventral masses of nerve tissue is reached. There are two ganglia 
on each side, a lateral pair somewhat smaller than the more ventral. The lateral 
are the pleural and the ventral are the pedal ganglia. The pedal ganglia are closely 
pressed against each other in the middle line, but not fused, they are much larger 
than any of the other ganglionic pairs and of a more complicated cell and fibrous 
structure. 
Beyond the region of large ganglia and slightly farther towards the other end 
of the animal, on the right side, a small visceral ganglion makes its appearance. 
Farther down on the left side a much smaller group of cells seems to indicate another 
ganglion of the viscera. 
(Contribution from the Zoological Laboratory of Pomona College) 
