254 SOME MINUTE ANIMAL PARASITES 



two clear polar capsules (Fig. 48). They are washed 

 out from the body of a dead fish in great numbers 

 and with ease ; and as the infected fish decays in the 

 water, so does the plague spread. When epidemics 

 have arisen, the only possible remedy has been to 

 net the stream where any dead fish has been seen, 

 and to destroy ruthlessly any one exhibiting the 

 slightest trace of skin tumours. Destruction by 

 killing is not enough ; the corpses must be burned, 

 and not buried or left to decay in the air, as the 

 spores could again reach the water under the action 

 of wind and rain. 



The disease described as " smallpox " (Pocken- 

 krankheit) of the carp is due to the action of 

 Myxobolus cyprini. The external appearance of the 

 infected fish resembles that of the barbel, but there 

 is a very great difference between the two sets of 

 tumours seen on the skin. In the barbel the tumours 

 consist of disintegrated tissue and numerous tropho- 

 zoites and spores of the parasite ; but the swellings 

 on the carp are secondary results of the action of 

 Myxobolus cyprini. They consist of an enormous 

 number of colourless blood-corpuscles which have 

 filtered into the epidermis and the layers beneath it, 

 and have formed a mass of tissue there, mingled 

 with some red cells of the blood. 



The young stages of the parasite occur in numbers 

 in the epithelial cells lining the kidney tubules, and 

 thereby interfere considerably with the excretory 

 function of these organs. They increase in size, and 

 subsequently rupture the epithelium, so reaching the 

 channel of the tubule, which soon becomes blocked 



