272 SOME MINUTE ANIMAL PARASITES 



a. 



days. It is then considered quite safe for use again. 

 An alternative method is merely to remove the fish. 

 Lack of food means death of the parasite, and as 

 before, three days are considered enough to purify 

 the tank. An outbreak in the open is far more 

 difficult to treat, but cases are known in which the 



disease was stamped 

 out by netting the 

 stream within certain 

 limits and preventing 

 the access of fish to 

 the part treated. 



So far, no mention 

 has been made of any 

 form of sexual mani- 

 festation, such as is 

 seen among other 

 Ciliates. Conjuga- 

 tion has, apparently, 

 not been seen. Ac- 

 cording to one 

 observer, nuclear 

 division occurs in 

 the young Ichthyoph- 

 thirius, whose micro- 

 nucleus divides into four. Of these, three perish 

 and the fourth one divides again. What happens 

 after this is somewhat uncertain. It is suggested 

 that the two nuclei reunite, and so bring about a 

 sort of self-fertilization or autogamy. 



Opalina (Fig. 53) is perhaps the ciliate showing the 

 greatest degradation due to parasitism in the group. 

 Though it is multinucleate, it is in many ways a 



FIG. 53 OPALINA FROM THE RECTUM 

 OF THE FROG 



a, Myonemes ; b, nuclei 



