THE TREMATODA. 85 



stand how the Cercarim which had attained their full 

 growth, escaped from the close-shut sacculi which the 

 " nurses" appeared to them to be ; and when the whole 

 swarm of Cercarice is seen moving about within the contain- 

 ing cyst, it certainly seems as if they were endeavouring to 

 find an outlet. I have no doubt but that the animal is 

 provided with such an outlet, since I have been able to 

 demonstrate in the younger " nurses' at least two open- 

 ings, mtliout however being able to determine the precise 

 function of each ; and I have besides so often seen a series 

 of Cercarice one after another escaping from the orifice at 

 the free extremity of the " nurse,'' that I have been in- 

 duced to consider that opening as their natural exit. The 

 correctness of this opinion would further appear from the 

 fact, that the above phenomenon is always observed to 

 occm* in the same spot, even when the animal has not 

 been subjected to the slightest pressure. That this birth, 

 if such an expression may be used, most frequently occurs 

 when any circumstance promotes the expulsion of the 

 brood from the " nurse' is natural, and its occurrence is 

 readily explained by a phenomenon which is at first sight 

 surprising, that thousands of Cercarice will suddenly be 

 seen swarming in a glass of water, when a snail placed in 

 it is compelled in some way or another to withdraw itself 

 quickly into its shell. As has been previously remarked, 

 when the " nurses" are compressed in this way by the 

 smTounding organs, the progeny is expelled and carried 

 together with the water forced out by the contraction of 

 the snail, through the canals, into the surrounding water, 

 in which we found them when we began to trace the his- 

 tory of their development. 



That such a half natural, half artificial birth is not the 

 lot of all Cercarice is evident, even from the observations of 

 Siebold, who has sometimes noticed loitliin the " nurses," 

 and among half developed Cercarice, some individuals 

 which having already cast off the tail, were in a con- 

 dition to assume the pupa state. I have had numerous 

 opportunities of confirming this observation and even of 



