325 



daughter-individuals, products of schizogony, are termed mero- 

 zoites, to distinguish them from sporozoites which they may resemble 

 closely. Sooner or later, however, the propagative phase, destined 

 to infect new hosts, makes its appearance ; so-called sporonts (see 

 p. 330, infra) multiply by uporogony, which is combined with 

 sexual phases, to produce the sporozoites. The life-cycle of the 

 parasite may be passed entirely in one host, or there may be an 

 alternation of hosts of different species, with a distinct series of 

 phases of the parasite in each. When there is but a single species 

 of host, the method of infection of new hosts is usually contamina- 

 tive (p. 24), by means of resistant spores and cysts ; when there is 

 an alternation of hosts, the infection may be inoculative (p. 23), 

 without resistant phases, as in malarial parasites, or contamina- 

 tive, with resistant phases, as in Aggregata (p. 353). 



Whether the life-cycle be of simple or complex type, it ends 

 with the production of sporozoites, bringing it back to the starting- 

 point again ; and in the vast majority of cases the sporozoites are 

 enclosed, one or more together, in tough sporocysts to form the 

 characteristic resistant spores. As a rule each spore arises from 

 a single spore-mother-cell or sporoblast. 



The Sporozoa fall naturally into two subclasses, which' have 

 received various names, according as one or another of their char- 

 acteristic features has been considered diagnostic. It is best to 

 define each subclass by a number of characteristics, since none by 

 itself is sufficiently distinctive. 



In the first subclass the trophic and reproductive phases are 

 typically distinct that is to say, the animal becomes full-grown, 

 and ceases to grow further, before reproduction begins, hence 

 Telosporidia (Schaudinn) ; reproduction takes place usually by 

 a process of multiple fission in which the daughter-individuals are 

 budded or split off on the outer surface of the parent-body, 

 hence Ectosporea (Metchnikoff) ; and the germs or sporozoites 

 produced are gregarinulae, hence Rhabdogeniae (Delage and 

 Herouard). 



In the second subclass the trophic and reproductive phases 

 usually overlap that is to say, the still-growing or even quite 

 young trophozoite may begin to form spores, hence Neosporidia 

 (Schaudinn) ; the spore-mother-cells are formed by a process of 

 internal gemmation, being cut off within the cytoplasm of the 

 parent, hence Endosporea (Metchnikoff) ; and the- sporozoites 

 produced are amoebulae, hence Amoebogeniae (Delage and 

 Herouard). 



Of the three contrasted characters by which the two subclasses 

 are distinguished, the most absolutely diagnostic is probably the 

 form of the sporozoite. The names Telosporidia and Neosporidia 



