MYXOSPORIDIASIS 237 



laminated meat can be partially avoided. But 

 a large proportion of the world's population is 

 not mainly meat-eating, and for them there are 

 dangers that are far more insidious. The peoples 

 who eat either little or no meat often partake freely 

 of fish, and fish are attacked by a variety of diseases, 

 some of which have few or no marked external 

 symptoms, while others possess them. Many such 

 diseases are due to minute Protozoa belonging to 

 the Myxosporidia, this group of organisms being 

 nearly allied to the Microsporidia. 



The Myxosporidia are extremely interesting. They 

 are small, though giants compared with the Micro- 

 sporidia, and, like the latter group, are widely dis- 

 tributed, for not only salt and fresh-water fish, but 

 Crustacea also are attacked. The barbel disease of 

 the South of England and Germany, and the trout 

 destruction and pike devastation of Germany and 

 many parts of France, are still events within the 

 memory of the present generation, and Myxosporidia 

 were the agents of extermination in each case. 



Myxosporidia, like Microsporidia, are sometimes 

 restricted to one organ of their host, while in other 

 cases they diffuse gradually throughout the body of 

 the animal infected. Naturally, the tissue parasites 

 have much the more obvious effects on their hosts, 

 but it is quite inaccurate to suppose that the para- 

 sites found within the body cavity, urinary bladder, 

 or gall-bladder of a fish, seemingly free-floating, are 

 non-injurious. Such is far from being the case, for 

 changes are introduced into the metabolism of the 

 animal, sometimes with very grave results ; and, on 



