294 EENEST WARREN. 



The occurrence of the sexless D. isostomum in the cray- 

 fish complicates the problem, and if Zaddach is correct in 

 regarding this sexless form as arising from the eggs of D. 

 cirrigerum, it would follow that either the sexual or the 

 sexless form can be produced in the crayfish according to 

 circumstances. In my numerous sections there occurred one 

 remarkable individual which cannot altogether be under- 

 stood. It was embedded in the superficial substance of the 

 testes, and apparently had developed in this position from 

 the egg. It was not enclosed in a cyst, and had no appre- 

 ciable cuticle. It differed from the usual form in having no 

 obvious trace of generative or copulatory organs. From a 

 solitary specimen found in section it is scarcely safe to 

 speculate, but its structure is certainly suggestive that it 

 would have developed into D. isostomum. 



The chief peculiarity in the structure of the species is the 

 remarkable excretory epithelium of the bladder. The epi- 

 thelium consists of very large cells which apparently are 

 capable of extracting waste matter direct from the surround- 

 ing parenchyma, since the excretory tubes are few and small. 

 The cells are able to bud; after a period of activity they 

 become effete and are passed out of the bladder. Ultimately 

 they disintegrate and add to the thickness of the cyst-wall. 

 Fresh cells are produced in the bladder from reserve nuclei 

 lying on the basal membrane. 



The animal grows to maturity permanently enclosed in a 

 cyst, hence the cyst-wall must be highly elastic and permeable 

 to nutritive fluids. I believe that most, if not the entire 

 substance of the cyst-wall, is derived from the worn-out 

 excretory cells. 



Laurer's duct comes off in close connection with the recep- 

 taculum seminis, as if for the purpose of keeping this vesicle 

 free from waste yolk and cement substance. The position 

 of the cirrus-sac varies considerably in different individuals. 

 I have not observed the act of self-copulation. 



According to Looss's system the present Distonuim may be 

 referred to the sub-family Lepodermatinae, and it is placed 



