MUSEUM OF COMPARATIVE ZOOLOGY. 161 
Although numerous fine processes pass off from the cell body in vari- 
ous directions, each cell has one prominent process, which may usually 
be followed without difficulty. ‘They are, hence, really unipolar cells. 
Of these cells the first and fifth pairs (Fig. 94) are much larger than 
the others, the third is intermediate in size, and the second and fourth 
are considerably smaller, though nearly equal in size to each other. All 
things considered, each of these cells has such a characteristic appear- 
ance that after study it is possible to recognize at once a cell from any 
pair. 
One also finds a few cells about half as large as those of the second 
class; they vary in position and seemingly in number in different speci- 
mens. A pair of these are shown in Figure 84 (Plate VI.) between the 
nuclei of the fifth pair of large cells. These cells are too indefinite in 
number and position to be regarded as constituting a third class. They 
resemble the cells of the second class in general appearance, differing 
from the latter only in size. At most one finds two pairs of such cells 
ventral to the fourth pair (Fig. 84), and another pair anterior to the 
third pair of large ganglion cells (Fig. 76). They constitute perhaps 
an appendix to the cells of the second class. 
It is necessary now to ascertain the exact position and relation of 
these cells to other parts of the nervous system. Figures 72 to 88 
(Plate VI.) represent a series of successive transverse sections including 
the entire brain. By comparing them with Figures 63 and 94, one may 
determine the exact position of the large cells, and follow their processes. 
In the cross sections only the nuclei are represented, since the cell body 
is too small and too poorly marked off from the surrounding tissue to 
be seen under this power. 
The first pair (Plate V. Fig. 63, cl. gn. 1.) lie farthest anterior, as 
well as most ventral of all. They usually approach the median plane 
of the body very closely, being separated from each other by only a 
narrow space. Occasionally one of them lies a little higher than the 
other in the fibrous mass of the brain. These cells are pear-shaped 
(Plate VII. Fig. 94), with the long diameter parallel with the chief 
axis of the animal. Each possesses a single large process, which passes 
directly backward. As the two processes from these cells pass poste- 
riad they approach each other and rise slightly, by which they come to 
lie in the central V-shaped portion of the ventral nerve cord (Plate VI. 
Fig. 88). 
The second of the five pairs of large cells is somewhat smaller than 
the first, and its position varies within narrow limits (Plate VII. Fig. 94). 
