REPORT UPON A. PARASITIC PROTOZOAN. 185 



Zacharias has stated that the auimal loses its cilia before encystrnent and has 

 expressed his belief that the cyst-membrane is the thrown-off cuticle. From the above 

 description it will be seen that my results do not agree with his. Admitting the pos- 

 sibility that the forms we have studied represent two different species, in which case 

 we might perhaps expect to find some differences in the mode of reproduction, etc., 

 I will repeat here that in the form Forbes and I have seen the cilia are not thrown 

 off either before or during encystment; that the cyst-membrane in this case is not a 

 discarded cuticle is shown conclusively by the fact that it appears at different stages of 

 division. The origin of the cyst-membrane is not clear to me. The only explanation 

 of its formation which occurs to me is that it is formed by an excretion of the cells 

 which through the constant rotation of the latter is gradually moved centrifugally, 

 condensing at a short distance from the organism. 



I was unable to directly observe the division of any specimen beyond the 128-cell 

 stage, as described above, but the division certainly goes beyond that stage, for not 

 only have former authors described and figured it, but I have also found cysts 

 containing more than 128 cells. Furthermore, the youngest stages found on the fish 

 are smaller than the cells observed in the 128-cell stage. 



In one case a cyst contained 4 cells at 3 p. m., 8 at 4:10, 1G at 4:25. At 11 a. m. 

 on the following day it corresponded to the stage given by Fouquet as the final stage 

 of division, and must have contained from 500 to 800, or possibly 1,000, minute cells, 

 the latter corresponding to the description of the young given by Fouquet, except that 

 I could not distinguish the trichocysts. 



These young were also found swimming around on the bottom of a glass in 

 which several trout had been confined, and were found in great numbers on the fish, so 

 there can be no doubt that this is the stage which serves to spread the infection from 

 fish to fish. It will be noticed that this stage was reached in less than 24 hours in the 

 specimen referred to. 



From the life-history as just given, in confirmation of the results obtained by 

 former authors, it is evident that, although the parasites may, in all probability, mul- 

 tiply by division while on the fish, yet they eventually leave their host, swim around 

 in the water for a time, become encysted, and run through successive divisions in the 

 geometrical series 2, 4, 8, 16, etc., and reinfect fish as a minute body, invisible to the 

 naked eye and about 30;< (estimated) long. As all experiments to kill the parasites 

 while on the fish were unsatisfactory, it is evident that we still have a mode of pre- 

 vention open to us in attacking the protozoan while it is free in the water. The problem 

 of treatment now resolves itself into finding an inexpensive solution in which the fish 

 can live until all the parasites have left the body, but the same solution must be fatal 

 to the protozoa. The duration of life of the individual protozoan is a factor of con- 

 siderable importance in this connection. It will be naturally expected that this will 

 be subject to some variation and that no definite estimate can be made of it. Nor is it 

 possible to directly observe how long one of the protozoa is able to live on the fish. In 

 one of my experiments, however, a catfisU was completely cleared of its parasites in 

 about a week's time, while a trout was cleared in ten days' time, and these figures may 

 form a basis for the estimation of the length of time it is necessary to subject the fish 

 to treatment. 



