CNIDOSPORIDIA, MYXOSPORIDIA, ACTINOMYXIDIA 303 



The spore is small; with one-piece membrane; one (rarely two) polar 



filament; polar capsule, if present, invisible in vivo 



Order 3 Microsporidia 



The spore small, barrel-shaped; a thick filament, coiled beneath the spore 



membrane; three sporoplasms Order 4 Helicosporidia 



ORDER 1 MYXOSPORIDIA BUTSCHLI 



The Myxosporidia are characterized by the possession of a 

 spore which shows the following structure: The spore is of 

 various shape and dimension. It is covered by a bivalve chitin- 

 oid spore membrane, the two valves meeting in a sutural plane 

 which is either twisted (in three genera) or more or less straight. 

 The membrane may possess various markings or processes. The 

 polar capsule, with its short coiled filament, varies in number 

 from one to four. Except in the family Myxidiidae, in which one 

 polar capsule is situated near each of the poles of the spore, the 

 polar capsules are always grouped at one end which is desig- 

 nated as the anterior end of the spore. Below or between (in 

 Myxidiidae) the polar capsules there is a sporoplasm. Ordi- 

 narily a young spore possesses two nuclei which fuse into one 

 when the spore becomes mature. In Myxobolidae there is a 

 glycogenous substance in a vacuole which stains mahogany red 

 with iodine and which is known as the iodinophilous (iodo- 

 phile) vacuole. 



The Myxosporidia are almost exclusively parasites of lower 

 vertebrates, especially fishes. Both fresh and salt water fishes 

 have been found to harbor, or to be infected by, Myxosporidia 

 in various regions of the world. A few occur in Amphibia and 

 Reptilia, but no species has been found to occur in either birds 

 or mammals. When a spore gains entrance into the digestive 

 tract of a specific host fish, the sporoplasm leaves the spore as 

 an amoebula which penetrates through the gut-epithelium and, 

 after a period of multiplication, enters the tissues of certain 

 organs, where it grows into a schizont at the expense of the host 

 tissue cells, and the nucleus divides repeatedly. Some nuclei 

 become surrounded by masses of cytoplasm and become the 

 sporonts (Fig. 128). The sporonts grow and their nuclei divide 

 several times, forming six to eighteen daughter nuclei, each 

 with a small mass of cytoplasm. The number of the nuclei thus 



