PROTOZOA IN GENERAL 



275 



red blood corpuscles of vertebrates. Again the life cycle involves 

 both schizogony and sporogony. The former occurs in the blood of 

 the vertebrate and the latter takes place in such hosts as insects, 

 leeches, and ticks. The malaria parasite and the causal agent of 

 Texas fever in cattle ar^ the most important forms. 



Cnidosporidia is the name of a second subclass, the spores of which 

 contain from one to four polar capsules each with a coiled polar fila- 

 ment. There are two orders: (a) Myxosporidia are found chiefly as 

 fish parasites, but occur occasionally in reptiles and amphibia. The 

 gall-bladder, uriniferous tubes, and urinary bladder are usual seats 

 of infection for the free forms, while the gills and muscles of the 

 fishes are choice tissues for the cysts. Myxidium and Myxoholus are 

 characteristic genera, (b) Microsporidia have in each spore a single 

 polar capsule. This group is parasitic chiefly in arthropods, and 

 occasionally in other invertebrates, fish and amphibia. 



Ep'itheUam 

 Early sta^e 



^.HOOr-C 



.Intermediate sta^c 



haturc Sta^e 



Fig. 91. — Gregarina attached to an epithelial cell of a host's intestine. Other stages 

 of its development are shown within adjacent cells. 



The third subclass Acnidosporidia includes forms which produce 

 simple spores. Again there are two orders: (a) Sarcosporidia. As 

 the name infers, these occur in muscles of several mammals. The 

 encysted forms attain a length of several millimeters, and ultimately 

 each becomes a mass of sickle-shaped spores. The complete life cycle 

 is not known, but the saclike, encysted form in muscles of mammals 

 is known as Miescher's corpuscle, (b) Haplosporidia are single cells, 

 each with a single nucleus, and they have a relatively simple struc- 

 ture. Individuals of this order parasitize fishes and certain insects, 

 notably the cockroach. 



Plasmodium, the malaria parasite, is one of the H aemosporidia, 

 and its life cycle will be given to illustrate the intricate life history 



