(_Contribution from the Zoological Laboratory of Pomona College_)
Specimens of this little gastropod mollusc 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 organization 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 buccal
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 connections 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.
[Illustration: Explanation of Figures]
Fig. 1 Camera lucida sketch of cerebral ganglia of Cæcum. The
dorsal side is up. ×300
Fig. 2. Left pleural ganglion of Cæcum. ×300.
Fig. 3. Left pedal ganglion of Cæcum. ×300.
Fig. 4. Reconstruction from Cæcum, showing position of eyes and
ganglia viewed from the ventral side. ×70.