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, d. gn. T.) 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 firsthand its position varies within narrow limits (Plate VII. Fig. 94). 



