24 Journal of Comparative Neurology. 



phuric has proven the best fluid. This fluid is compounded in 

 the following way : 



70 per cent. alcohoL . . . 100 vols. 



Sulphuric acid, cone. .... 2 vols. 



Picric acid, as much as will dissolve. 



For the above formula I am indebted to Dr. J. Play fair 

 McMurrich, now of Ann Arbor. Since this fluid sometimes 

 causes thin shells to curl, it is not so good for museum speci- 

 mens as 70 per cent, alcohol ; but for histological work it is ex- 

 cellent. It does not injure the tissues, it penetrates chitin easily, 

 and it fixes the cell structure. The living specimens were 

 placed in this fluid and allowed to remain for at least twenty- 

 four hours. Then after being washed in 70 per cent, alcohol 

 until all the picric acid had been removed, they were either 

 transferred to the stain or else, after being hardened and sec- 

 tioned in the usual way, they were stained on the slide. 



Among the numerous stains tried were: Kleinenberg's hae- 

 matoxylin, Delafield's haematoxylin, Czokor's alum cochineal, 

 borax caimine, eosin, etc., but none of these gave satisfactory 

 results. Finally Ehrlich's hematoxylin and tincture of alum 

 cochineal were tried. Both of these stains gave good results. 

 In using both of these stains the specimens were stained in toto. 

 When Ehrlich's haematoxylin was used, the specimens were over- 

 stained and then washed out with acidulated 70 per cent, alco- 

 hol ; but when the tincture of cochineal was used, after remain- 

 ing in the stain for about twent}'-four hours, the specimens were 

 washed in 70 per cent, alcohol and then hardened and sectioned 

 in the usual manner. 



PRELIMINARY. 



Although a lengthy discussion of the histology of extra- 

 neural systems would be foreign to the purpose of this paper, 

 yet, in order to facilitate a comprehensive description of the 

 nervous system, it is thought best to give a brief description of 

 the internal parts of the Ostracoda. 



All previous writers to the contrary notwithstanding, the 

 shell of the Ostracoda consists of three layers : an ectal layer, 



