282 ERNEST WARREN. 



crayfisli. In this condition the animal is bent on itself to 

 form a globular body^ and the ventral surface is outermost 

 (Fig. VII). The cyst-wall is of variable thickness, and its 

 mode of formation is peculiar. In sections through the 

 encysted animal there can be seen in the spaces between the 

 parasite and the cyst-wall rounded cells (Figs. A^II, VIII, 

 ex. cl.) containing two or more nuclei; these cells have passed 

 out from the excretory bladder. The loose cells disintegrate 

 in time into a reticular nodulated material which stains 

 deeply (/. mt.). The parasite is constantly rotating inside 

 its cyst, and the disintegrated substance, or even the cells 

 themselves before breaking up, are gradually flattened 

 against the inner surface of the cyst, and increase its thick- 

 ness (Fig. VIII, 2, ex. cl.). It is probable that the greater 

 part, if not the entire substance of the cyst-wall is built up 

 out of such discharged excretory cells. 



The cyst-wall frequently exhibits a distinct stratification 

 (Fig. VIII (2), st.'^-'^). The middle stratum {st.") appears 

 to be the densest, and it stains more intensely than the other 

 two layers. 



It would be exceedingly interesting to know whether 

 generally the cysts of trematodes which are permanently en- 

 cysted in the adult state originate in a similar manner. The 

 cysts so often produced by cercaria, and that formed in the 

 developing stages of D. cirrigerum, are clearly not formed 

 in this way ; they are true cuticular cysts formed by the 

 activity of certain cystogenous cells. It is very possible, 

 indeed, that such a true cuticular cyst would be unfitted for 

 enveloping a growing animal, it not being sufficiently elastic 

 or permeable to nutritive fluids. 



The cells of the bladder bud in a remarkable manner 

 (Figs. IX, X), and to a certain extent resemble the yellow 

 cells of the intestine of the earthworm. The structure of the 

 cytoplasm varies considerably in the different individuals 

 examined; probably this is due to the bladders being in 

 different physiological conditions at the time of fixation. 



