270 SOME MINUTE ANIMAL PARASITES 



heavily infected case the pustules run together, and 

 form large sores all over the body. 



The life- history of the parasite is not very simple. 

 The young parasite is very small ; it moves rapidly 

 by cilia in the water for a short time, but is not able 

 to live independently for long. Failing to obtain a 

 host fish, it perishes. If a fish passes near the place 



C. 



FlG. 52 ICHTHYOPHTHIRIUS 



A , A young trophozoite with oval macronucleus and small micro- 

 nucleus, and contractile vacuole ; B, full-grown individual, 

 showing large macronucleus, well-marked myophan striations, 

 cilia, terminal cell mouth ; C, a cyst containing numerous 

 young parasites about to escape 



where the tiny ciliate is swimming, the young parasite 

 drops on to it and fastens itself to the skin. There 

 it may remain, or it may force its way downwards to 

 the deeper layers, so that, in time, a little wall of skin 

 is formed around it, and the parasite lies in a pit 

 formed by the irritated skin, and may be entirely 

 enclosed by it. Growth continues at the expense 

 of the host, giving rise to the pustules. When the 



