THE FLAGELLATA 201 



case with the flagella, the reservoir, cytostome and other 

 organellae of one gamete are absorbed, but of the other persist 

 during and after conjugation. 



In most flagellates conjugation is followed by encystment, 

 but Copromonas is exceptional in this respect, that some of the 

 zygotes retain their flagella and other organellae, continue to 

 lead an active existence, and after the lapse of a certain time 

 begin to reproduce themselves asexually by longitudinal 

 division. It would appear, however, that sooner or later the 

 progeny of these scissiparous zygotes pass into the encysted 

 condition. The more normal course is for the zygote to 

 encyst itself shortly after conjugation. In this case it becomes 

 more rounded in shape, and decreases in size. The flagellum 

 is gradually drawn in, the movements of the animalcule cease 

 and a cyst-wall is formed round its body. At first the cyst- 

 wall is very delicate and not easily seen, but afterwards it 

 becomes thicker and of a gelatinous consistency, and even- 

 tually forms a hard resistant coat. 



As living Copromonads are not found in the rectum of 

 the frog, but may invariably be obtained from a suitable 

 culture of the faeces, the life history of the species is evidently 

 as follows. The resting cysts are scattered in damp places, 

 are washed down into pools and streams, and are casually 

 swallowed by a frog or toad along with its food. During their 

 passage through the alimentary tract the resistant cyst-walls 

 are dissolved, and on arriving in the rectum the monads are 

 ready to escape. When the faeces are passed to the exterior 

 and diluted with water (for they are sure to be deposited in 

 damp places) the monads emerge and for a time live in the 

 diluted medium suitable to their active existence and multiply 

 rapidly by division. But from the nature of the case the 

 conditions favourable to their active existence cannot last 

 long. The monads therefore conjugate, and the resulting 

 zygotes encyst. The cysts are scattered, are again swallowed 

 by a frog or toad and thus the cycle is completed. But if a 

 zygote should be placed in conditions favourable to active 

 existence it has the power of continuing to feed and multiply 

 by division until finally its progeny encyst and pass on to the 

 normal cycle of the life history. 



Polytoma uvella is a flagellate often found in long-standing 



