ANIMAL PARASITES. 131 



cycle within single forms of cells, passing then to other cells or to the 

 body cavities or to hosts of a different species. The life cycle of many 

 forms is extremely complex. On account of their strict parasitism and 

 the requirements in some instances of an interchange of hosts, precluding 

 the methods of culture applicable to many of the lower organisms, the 

 life history of many forms is still unknown or obscure. 



The classification of the Sporozoa is still tentative, but one may con- 

 veniently recognize the following orders: 



1. Gregarince. These are round or elongated parasites, some of the 

 higher forms presenting partitions in the cell with special development 

 of one end for attachment. They are parasitic in certain cold-blooded 

 animals, especially the iuvertebrata. The young stages only are intra- 

 cellular, mature forms occurring in the body spaces. 



2. The Myxosporidia are parasitic in certain of the invertebrates, in 

 fishes and batrachians. Epidemics among silk worms incited by a para- 

 site of this class have occasioned serious losses. Many species are con- 

 cerned in diseases of fish, in which they may cause extensive deep foci of 

 necrosis and ulceratiou. Cytoryctes variolse, believed to be the organism 

 inciting smallpox (see p. 275), is described as allied to this order of pro- 

 tozoa. 



3. Coccidia. Organisms of this order are parasitic in certain of the 

 invertebrates, in birds, reptiles, and mammals. They are round or oval, 



FIG. 56. COCCIDIUM OVIFORME. 



This shows the encapsulated form of the parasite with the formation of spores. 



usually intracellular parasites having no free motile adult stage. They 

 are most frequently found in the epithelium of the intestine and liver. 



One of the most common forms in the mammalia is Coccidium oviforme 

 which is of frequent occurrence in the liver of the rabbit, forming a 

 part of the contents of yellowish irregular shaped masses, resembling 

 tumors, or in the form of cysts. 



The parasites surround themselves with a capsule within which elon- 

 gated sporozites develop. This encapsulated form may be taken up by 

 a new host in which the sporozites are set free and enter the epithelial 

 cells in which they again become encapsulated. 



The occurrence of Coccidium oviforme has been recorded in the liver, 

 kidney, and heart-muscle of man. 



Another smaller form, occurring in the intestinal epithelium of dogs, 

 cats, and rabbits, has been found in two cases in a similar situation in 

 man. 



