648 PROTOZOOLOGY 



division reveals 4 chromosomes (6); within the trophozoite macro- 

 gametes and microgametes are independently formed, during which 

 process, chromosome number is reduced into half (2) (c, d) ; plasog- 

 amy between a macrogamete and a microgamete results in produc- 

 tion of a binucleate pansporoblast (e,f), from which repeated nuclear 

 division (g-l) forms 2 spores (m) ; each of the 2 nuclei of the sporo- 

 plasm is haploid and the diploid number is restored when the 2 nuclei 

 fuse into one (n). 



The site of infection by Myxosporidia varies among different 

 species. They have been found in almost all kinds of tissues and 

 organs of host fish, although each myxosporidian has its special site 

 of infection in one to several species of fish. The gills and gall bladder 

 are most frequently parasitized by Myxosporidia in freshwater 

 fishes, while the gall bladder and urinary bladder of marine fishes 

 harbor one or more species of Myxosporidia. When the infection is 

 concentrated in the fins or integument, the resulting changes are 

 quite conspicuous (Fig. 278). The infection in the gills is usually 



Fig. 278. A channel cat, heavily infected with Henneguya exilis, 

 X* (Kudo). 



manifest by whitish pustules which can be frequently detected with 

 the unaided eye. When the wall of the alimentary canal, mesentery, 

 liver, and other organs are attacked, one sees considerable changes 

 in them. Heavy myxosporidian infection of the gall bladder or uri- 

 nary bladder of the host fish may cause abnormal appearance and 

 coloration or unusual enlargement of the organ, but under ordinary 

 circumstances the infection is detected only by a microscopical ex- 

 amination of its contents. Certain histological changes in the host 

 fish have been mentioned elsewhere (p. 31). 



Severe epidemic diseases of fishes are frequently found to be due to 

 myxosporidian infections. According to Davis (1924), the "wormy" 

 halibut of the Pacific coast of North America is due to the myxo- 

 sporidian, Unicapsula muscalaris (Fig. 280), which invades the mus- 

 cular tissue of the host fish. The "boil disease" of the barbel, Barbus 

 barbus and others, of European waters, is caused by Myxobolus 

 pfeifferi (Keysselitz, 1908). Myxosoma cerebralis, which attacks the 



