324 RESEARCH IN PROTOZOOLOGY 



i6. Problems concerning seasonal occurrence. The great major- 

 ity of MYXOSPORIDIA were discovered during the warmer months 

 of the year. This is easily understood as the material is more abun- 

 dantly obtained and studied during this period. Do myxosporidia 

 occur in equal abundance in winter months as they do in summer 

 months? Gurley (1894) wrote that Myxobolus transovalis which 

 was first noted in June, could not be found in August in host fish 

 which were collected from the locality where the previous collec- 

 tion had been made. Davis (1917) stated similarly that Cerato- 

 myxa monospora was abundantly found in June, much less in July 

 and disappeared completely from the host fish at the end of the 

 month. Contrary to the above observations, Keysselitz (1908) 

 observed that the infection by Myxobolus pfeifferi as manifested 

 by the appearance of the boils on the barbel is not noticeable dur- 

 ing winter and spring months. The boils appear in April and 

 continue to be present up to the end of October, during which 

 months the majority of the heavily infected fish die. The highest 

 mortality is reached in the hottest months, i.e., July and August. 

 That the temperature affects the growth of the parasite was indi- 

 cated by a rapid daily growth in size of the boils on fish which 

 were kept in the aquarium at a temperature of 25° C. or higher. 



Wegener (1910) observed Henneguya psorospermica through- 

 out the year, but rarely during winter. According to Nemeczek 

 (1911), the cysts of Henneguya gigantea do not, after October, 

 contain the spore, but vegetative forms only. This observer holds 

 as does Keysselitz that the development of the spore depends ap- 

 parently upon the temperature of the water in which the host fish 

 live. Zandt (1924) studied Perca ^uzdatilis of Lake Constance, 

 Switzerland, for two years with regard to the infections by Henne- 

 guya similis. He found an annual periodic appearance of the 

 myxosporidian. The infection was first noted in December and 

 January, when mature spores are not found. The spores begin to 

 appear from February to March and infected fish were observed 

 up to about the end of May. From the latter time to December, no 

 host fish carrying cysts were recognized. 



These data are too meager to allow any definite conclusion. 

 There seem to be numerous factors affecting the host-parasite 

 relation in myxosporidia as revealed by these apparently contra- 

 dictory observations. In the bibliography, references which are 

 listed in Kudo (1920) are omitted. 



