294 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



America.* So far as I know, it has not previously been 

 observed in British, waters. The parasite is always 

 described as inhabiting the skin of the infested fish, 

 where it excavates little cavities or galleries or forms 

 pustules. These cavities or pustules may contain one or 

 a number of individual parasites. A characteristic 

 spotted appearance is thus conferred on the skin of 

 the fish. Ichthyophthirius reproduces either by fission 

 (Stiles), or by the method of encystment. Eventually the 

 parasite leaves the host, and after swimming about in the 

 water for some time surrounds itself by a cyst wall, when 

 it falls to the bottom. The young ciliated individuals 

 leaving the cyst then pass through a free-swimming stage 

 and reinfect fresh hosts. It is by attacking these free- 

 swimming forms that the epidemic in a tank or small pond 

 can most easily be fought. It appears to be impossible to 

 destroy the parasites while living in the skin of the hosts, 

 but if the fishes be taken from the aquarium and common 

 salt added to the latter with a copious supply of fresh 

 water the free-swimming individuals are easily killed. 

 This treatment, practicable though it may be on a small 

 scale, is, however, difficult in the case of a very large 

 pond or " lake." 



It is difficult to say what the specific pathogenic 

 action of the parasite may be. In the roach from Hesketh 

 Lake there was no general infection of the skin — indeed 

 I could not detect the parasite there in any of the 

 specimens examined. Even in the gills the numbers were 

 few, and it seemed difficult to believe that the mechanical 

 destruction of the tissues of the filaments was enough to 

 account for the death of the fish. It was, however, 



* There is an account of the parasite, with a summary of the 

 literature by C. W. Stiles in Bull. U.S. Fish. Comm., vol. 13, 1893, 

 pp. 173-190. See also Hofer. Handbuch der Fischkrankheiten, 

 p. 122 ; Munchen, 1904. 



