DISTOMUM CIRlilGERUM. 283 



The cytoplasm may present a more or less granular appear- 

 ance with a few globules of blackened fat. 



In other specimens the cytoplasm is richly vacuolated., and 

 the walls of the vacuoles stain intensely. This gives the 

 appearance of sharply defined, deeply stained curved fibrils 

 embedded in a homogeneous stroma. Only by very careful 

 observation under high powers have I been able to be con- 

 vinced that the apparent fibrils owe their existence to the fact 

 that the walls of the vacuoles stain with unequal intensity 

 (Fig.X(l),/.). 



Sometimes the vacuoles are of a larger size, and may be 

 seen to contain a granular substance (Fig. X, (3) f.p v.^)' In 

 other bladders every large cell contains one or more very 

 large vacuoles filled with the same granular material (4, r..,) ; 

 doubtless they are formed by the fusion of the smaller 

 vacuoles. Lastly, excretory sacs occur where the cells have 

 the large vacuoles more or less completely empty, and the 

 contents have apparently passed out into the cavity of the 

 bladder (5, t*. J . The globules of fatty matter appear to be 

 more abundant at the time the large vacuoles are formed, but 

 they are not confined to this period (Fig. X, cf. Band 43). 



The cells of the bladder would seem to proliferate rapidly 

 both by budding and by the upgrowth of fresh cells from the 

 basal membrane ; they become quickly effete, probably by 

 being overcharged with waste nitrogenous substances which 

 they are unable to eliminate sufficiently quickly. The cells 

 break away from the basal membrane and lie loose in the 

 cavity of the bladder ; from there they are squeezed out by 

 the contractions of the excretory bladder into the space be- 

 tween the animal and cyst. They now disintegrate, and are 

 plastered against the inner surface of the cyst to increase the 

 thickness of the cyst-wall. 



The whole of the phenomena must be regarded as the utili- 

 sation of waste nitrogenous matter, and this is analogous to 

 the vicAv that the exo-skeleton of Arthropods is ultimately 

 derived from such waste. 



