LIFE OF ICHTHYOPHTHIRIUS 271 



organism is fully grown, it forces its way to the 

 surface and bursts through, leaving a small, gaping 

 hole behind. Sometimes a mass of skin is torn away 

 with it, producing larger sores, which afford a means 

 of entry to Saprolegnia and other fungi so destructive 

 to fish. The nearly globular parasite meanwhile 

 sinks to the bottom of the stream or tank, and rests 

 on the bottom or on pondweed. Multiplication 

 then commences. The Ichthyophthirius first forms 

 a thin gelatinous cyst for itself, and then its own 

 body substance divides into two parts, which separate 

 from one another within the cyst. The division is 

 repeated, the number of divisions varying according 

 to the size of the parent when it left the fish. 

 As many as 256 daughter forms have been pro- 

 duced, but less than that number is not uncommon 

 (Fig. 52, C). Sometimes individual cysts show 

 peculiarities such as one only of the two parts 

 produced by the first division continuing to divide. 

 The cyst finally becomes much softer, and the 

 young parasites swarm out from it and await the 

 arrival of a fish, such as a trout or salmon, which 

 may serve as a new host. 



When the young Ichthyophthirius leaves the cyst, 

 it is about ^ mm, long, and it lives as a free- 

 swimming organism for a short time. Should that 

 time be prolonged, the young parasite dies. 

 Advantage is taken of this fact in attacking out- 

 breaks in aquaria. Fish known to be diseased are 

 destroyed, and the seemingly healthy stock removed. 

 The water is then drained from the infected tank, 

 and the latter is exposed to light and air for three 



