454 PROTOZOOLOGY 



Order 1 Myxosporidia Biitschli 

 The spore of a myxosporidian is of various shapes and dimen- 

 sions. It is covered by a bivalve chitinous 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 ordinarily designated as the anterior 

 end of the spore. Below or between (in Myxidiidae) the polar cap- 

 sules, there is almost always a sporoplasm. Ordinarily a young 

 spore possesses two nuclei which fuse into one (autogamy) 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 (iodophile) 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 Rep- 

 tilia, 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 amoe- 

 bula 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. 208). The sporonts grow and their nuclei divide several 

 times, forming 6-18 daughter nuclei, each with a small mass of 

 cytoplasm. The number of the nuclei thus produced depends upon 

 the structure of the mature spore, and also upon whether 1 or 2 

 spores develop in a sporont. When the sporont develops into a 

 single spore, it is called a monosporoblastic sporont, and if two 

 spores are formed within a sporont, which is usually the case, the 

 sporont is called disporoblastic, or pansporoblast. The spore- 

 formation begins usually in the central area of the large tropho- 

 zoite, which continues to grow. The surrounding host tissue be- 

 comes degenerated or modified and forms an envelope which is 

 often large enough to be visible to the naked eye. This is ordi- 



