246 SOME MINUTE ANIMAL PARASITES 



trophozoites, growing and feeding passively within 

 the bladder of the host. Reproduction of this form 

 is termed plasmotomy, and has been best investigated 

 in the genus Myxidium. The Polysporea can be 

 discussed best in three main groups, and certain 

 members of these groups are responsible for many 

 destructive outbreaks among fish, both fresh-water 

 and marine. 



The first group, the Myxidiidae, are fairly near to 

 the Ceratomyxidse. The Myxidiidae are more par- 

 ticularly known as cavity parasites, though some 

 occur in tissues. They cannot be said to be highly 

 pathogenic to the host, nor in many cases are there 

 external indications of the occurrence of the parasite 

 within. The parasites are separated into two main 

 genera : (i) Myxidium, with several species, and 

 (2) Sph&rospora. Both are alike in having a very 

 distinct ectoplasm and endoplasm, the latter being 

 highly charged with granules, some of which are 

 ectoplasmic, while others are yellowish and are 

 probably excretory in nature. 



The parasite Myxidium lieberkuhni, from the urinary 

 bladder of the pike, is one of the best known. The 

 pathological effect of the parasite upon its host does 

 not seem to have been investigated as completely as 

 it needs. The trophozoites are large and propagate 

 by direct budding (plasmotomy), as before mentioned. 

 Many examples can be found that never show spores, 

 but old specimens sometimes show numbers of the 

 somewhat spindle-shaped spores which possess a 

 capsule at each end. They are 1 8 to 20 //, long, and 

 5 to 6 fi broad. 



