252 SOME MINUTE ANIMAL PARASITES 



relatively harmless parasite when it was found in 

 cavities such as the gall-bladder. As with other 

 members of the Myxosporidia, the belief in its 

 harmlessness was misplaced, for its presence causes 

 digestive derangements, renders the wall of the 

 bladder very much thicker and less elastic, and by 

 interference with nutrition causes emaciation, as 

 shown by loss of weight. 



The Myxobolidae are probably the family of the 

 Myxosporidia to be most dreaded by the fish-stocker 

 and sportsman. Many of the members are tissue 

 parasites, and are pathogenic to the animals harbour- 

 ing them. Sometimes they form large masses that 

 block the kidney tubules of their hosts ; or they may 

 form diffuse masses penetrating throughout the body 

 of the victim. They have very varied forms of 

 trophozoites, much depending on the size and shape 

 of the cavity in which the parasite is found. Simi- 

 larly, the number of spores produced by any one 

 trophozoite varies; but it is characteristic of the 

 Myxobolidae that they form two pear-shaped polar 

 capsules at one end of the spore. 



The genus Myxobolus is more particularly a para- 

 site of fresh-water fishes. The pike, carp, tench, 

 barbel, trout, roach, and chub, among others, are 

 badly infested, and the barbel has become practi- 

 cally extinct in many waters as the result of the 

 infection with a Myxobohis. In the case of the 

 barbel, the carp, the trout, the roach and chub, there 

 is external evidence of the disease in the form of 

 numerous and tumour-like swellings on the skin. 

 These may be relatively soft, or, as in Cypnnodon 



