NERVOUS SYSTEM OF CRUSTACEA 437 



difficulty in seeing all at one focus, difference in mounting and 

 difference in staining, yet the following seems clear: 



1. The cells often differ slightly in number in different similar 

 ganglia of the same animal, both in the same segment on each 

 side and in different segments at various levels. 



2. The number of cells is also variable in the same parts of the 

 same ganglia in different animals. 



3. The peripheral parts supplied by each of these ganglia do 

 not differ in any way that could be determined. 



4. It was even more clear that the cells in the commissures 

 differed widely. The next to the last cephalic commissure in one 

 specimen had 38 cells, the next 20, then, 19, 7, 10, 12, 16, 10, 

 etc. Similar variations were found in other specimens. The 

 lower cephalic commissures as a rule had more cells, while the 

 upper had less. 



It was also noted that some of the large cells which have quite 

 a characteristic position are in some places represented by one 

 cell, in others by two. In a few cases noted the large cells 

 have an independent peripheral distribution as compared with 

 the usual indirect distribution through a commissure. It is as 

 though a cell which ordinarily grew out to the periphery by way 

 of a commissure missed it in some way and left the ganglion by 

 a single fiber. 



FIBER TRACTS 



Branchipus was especially studied because the material was 



more favorable. 



The brain so far as could be determined is united from side to 



side by four commissures, a dorsal, two medial and a ventral. 



The last is below a small group of medial cells. The commissure 



just above this group seems the largest. The ventral commissure 



is partly from near-by cells and from basal parts of the con- 

 Fig. 9 Sixth and fifth ventral ganglia of Branchipus. Surface view. Cephalic 



side at the top. X 75. 



Fig. 10 Section through one abdominal ganglion of Artemia. The dorsal 



side is above. X 75. 



Figs. 11, 12 and 13 Sections through various levels of the brain of Artemia, 



from the base to the region of the optic nerve. The dorsal side is up. X 75. 



