1917] The Nervous System of Thysanura 307 



GENEIL\L CHARACTER OF THE BRAIN. 



One of the chief differences between the brains of Campodea 

 and Evahjapax as compared with Lepisma and MachiUs is due 

 to the lack of eyes in the first two. There are numerous differ- 

 ences between the brains of the first two genera. The shape 

 of the brain of Campodea is given as it appears when viewed 

 from above in Figure 5. Longitudinal and cross sections 

 through the brain show the ventral parts largely without cells. 

 (Figures 23-26). 



The cephalic and mid-dorsal regions are not so well supplied 

 as the lateral dorsal and caudal regions ; the caudal lateral region 

 of the brain has the largest mass of cells. Many fibers run 

 from the brain, from or to, forward, median or lateral parts 

 down the connectives to the subesophageal ganglion and 

 farther. There are also numerous small bundles which connect 

 all parts. In the latero-caudal region there are central masses of 

 denser fibers. Three well-marked masses at least may be 

 seen on each side near the dorso-lateral region. Fig. 23. These 

 may represent the areas which in other species help form the 

 mushroom bodies. The brain of Evalljapax is shown from 

 above in Fig. 19. On the right side is shown the position of 

 the tracheal tubes of that side and on the other the position of 

 the larger cell areas as shown in the methylene blue preparation. 

 The shape of the ganglion is quite different from that of Cam- 

 podea, as the drawing indicates. There are cells on the dorsal 

 side of the brain, but they are few compared to the other 

 great areas indicated in the drawing. In section the brain 

 seemed simpler in structure than that of Campodea, but 

 this in part may have been the fault of the preparation. As in 

 Campodea, the ventral regions of the brain are without cells. 

 No clear indication of mushroom bodies was seen, and the 

 connections between different parts of the brain and the con- 

 nectives and subesophageal ganglion seemed less marked. 



Machilis and Lepisma also differ from each other to a marked 

 degree in brain structure and arrangement. The general 

 distribution of cells above and below is shown in two figures, 

 12 and 13, which were stained in methylen blue. In an adult 

 there seem to be not as many cells in proportion to the general 

 area of fibers as in some of the other genera. The middle line 

 both above and below is largely without cells as shown in Fig. 32, 



