278 RESEARCH IN PROTOZOOLOGY 



are developed each containing four sporozoites. A single coccidium, 

 Isospora hominis, is known with certainty to occur in man. It 

 appears to be rare. Very little is known about this species except 

 the development within the oocyst, hence we must assume the 

 rest of the life-cycle of this species from what we know of the 

 life-cycle of species belonging to the same genus that occur in 

 lower animals. Dobell (1919) named several species of coccidia 

 from man which were later proved by Thomson and Robertson 

 (1926a, 1926^) to be parasites of fish, the oocysts of which had 

 been eaten by the human host, had passed through the intestine 

 and been found in the feces. The oocysts of the coccidia are very 

 resistant and it is thus necessary for one to be certain that he is 

 dealing with an actual parasite of any particular host and not 

 with a foreign parasite that has simply passed through the alimen- 

 tary canal. Cross-infection experiments, especially by Andrews 

 (1927), have yielded many interesting facts regarding the host- 

 parasite specificity of coccidia. Many problems are open for study 

 involving the host-parasite relations of members of this group. 

 (See Chapter XXXI.) 



H^MOSPORIDIA 



Among the h^mosporidia are a number of families and genera 

 of great interest. The genus Plasmodiuui which contains the 

 malarial parasites will be treated in general in Chapter XXXIV. 

 Species belonging to other genera that may be obtained without 

 great difficulty are as follows : 



The genus Hcumoproteus occurs in the endothelial cells of the 

 blood vessels or in the red blood cells of vertebrates. Many species 

 of common birds are infected with Hccinoproteus. The best known 

 species is H. colmnbcc. In tropical and semitropical regions this 

 species is transmitted from pigeon to pigeon by a hippoboscid fly 

 of the genus Lynchia. How Hcemoproteus is transmitted in regions 

 where Lynchia does not occur is not known. The life-cycle of 

 Hcemoprotcus has been described especially by Adie (1915, 1924) 

 but needs further study. Many attempts have been made to trans- 

 fer infections from one vertebrate host to another by blood inocula- 

 tions without success. If an easy method of transfer could be 

 devised a number of problems in the host-parasite relations of 

 Hcemoprotcus could be attacked with profit. 



To the genus Lcucocytozoon belong parasites that occur in the 



