1 66 THE SPOROZOA 



trophozoite, and spores are formed continually during the trophic 

 stage, so that there is no distinction between trophozoite and 

 sporont. Hence it has been proposed by Schaudinn to group the 

 Gregarinida, Coccidiidea, and Haemosporidia together as a sub- 

 class Telosporidia, contrasting them with a sub -class Neosporidia 

 comprising the Myxosporidia and Sarcosporidia. The Telosporidia 

 are Sporozoa in which the reproductive phases follow completion 

 and cessation of growth ; the Neosporidia are Sporozoa in which 

 growth and reproduction go on at the same time. It is probable 

 that this distinction indicates the deepest phylogenetic cleft in 

 this class of Protozoa. 



(2) In Monocystis the whole life -history is a single cycle, 

 adapted entirely to spreading the infection amongst new hosts; 

 it is, in fact, monogenetic. But in many other Sporozoa, belonging 

 to either of the two sub-classes recognised above, the parasite may 

 be capable of rapid multiplication within the body of its host, 

 which it thus completely overruns in many instances. In such 

 cases the life-cycle becomes digenetic, that is to say, it is differentiated 

 into two distinct generations or series of generations, the one endo- 

 genous or self-infective, the other exogenous or cross-infective. In 

 the endogenous generations the reproductive processes are usually 

 of a primitive type, taking place by binary or multiple fission, or 

 by a simple form of sporulation, known as schizogony, in which a 

 trophozoite, without encystation, breaks up into numerous gymno- 

 spores, implanted on a certain amount of residual protoplasm. The 

 sporulating individuals in this case are termed schizonts, and the 

 gymnospores are known as merozoites, to distinguish them from 

 the sporozoites of the exogenous generation. After a number of 

 endogenous generations, the parasite soon or later reproduces itself 

 by exogenous generation or sporogony, with the formation from 

 sporonts of resistent spores that can be disseminated outside the 

 body of the host. In monogenetic types the life-cycle consists of 

 sporogony alone. 



(3) Considerable differences are seen in the manner in which 

 the infection of a new host is brought about. The vast majority 

 of Sporozoa appear to be disseminated passively, and the spores 

 are taken up directly, in an accidental manner, by another host of 

 the same kind as that from which they came. Should the spores 

 chance to be devoured by an animal of another species, they will 

 either be digested completely or will pass through its body un- 

 altered. Only in their proper host do the digestive juices have 

 the effect of liberating the sporozoites without harming them. In 

 some cases, however, especially amongst Sporozoa parasitic in the 

 blood, an intermediate host has been acquired, and is the agent 

 by which the parasite is disseminated. The best-known instance 

 of this is found in the malarial parasites, and is fully described 



