324 THE PROTOZOA 



having nothing in common except the parasitic habit and the 

 adaptations arising from it, more especially the propagation by 

 spores. 



The modern tendency is rather to split up this vast assemblage 

 into smaller groups, and to abolish the Sporozoa as a primary 

 subdivision of the Protozoa. It is practically certain, at least, that 

 the two main subclasses into which it is always divided are per- 

 fectly distinct in their origin. The class Sporozoa is retained here 

 solely in deference to custom and convenience, and without preju- 

 dice to the affinities and systematic position of its constituents, 

 a question which will be discussed when the group as a whole has 

 been surveyed. 



The life-cycle of a Sporozoon may be started conveniently from 

 the minute germ or sporozoite which escapes from the spore, or 

 from the corresponding stage when spores are not formed. The 

 sporozoite may have one of two forms : it may be an amoebula, 

 a minute amoeboid organism ; or it may be of definite form, a little 

 rod-like or sickle-shaped animalcule (" falciform body," " Sichel- 

 keim ") which is capable of twisting or bending movements, but 

 retains its body-form, and progresses by gliding forwards ; for this 

 second type of sporozoite the term " gregarinula " has been proposed 

 in a previous chapter (p. 169). 



The sporozoite, whatever its form, is liberated in the body of 

 the new host, and begins at once its parasitic career ; it nourishes 

 itself and grows, often to a relatively huge size, at the expense of 

 the host. This phase of the life-history is termed the " trophic 

 phase," and the parasite itself during this phase a trophozoite, 

 by which term is understood a parasite that is actually absorbing 

 nourishment from the host. The trophozoite may be lodged within 

 cells (cytozoic), or in tissues of the body amongst the cells (histo- 

 zoic), or in some cavity of the body in which it either lies free or 

 is attached to the wall (ccelozoic). Whatever their habitat, the 

 trophozoites of Sporozoa never exhibit any organs or mechanisms 

 for the ingestion or digestion of food, but absorb their nutriment 

 in all cases in the fluid state, by osmosis through the surface of 

 the body, from the substance of the host ; if pseudopodia or flagella 

 are possessed by these parasites, they are never used for food- 

 capture, except in so far as pseudopodia, by increasing the surface 

 of the body, may augment its absorptive powers. 



The parasite may exhibit multiplicative phases in which it 

 reproduces itself actively, so that there may be many generations 

 of trophozoites within one and the same host, which may thus be 

 quite overrun by swarms of the parasites. Multiplication of this 

 kind, which is non-sexual, is known as schizogony ; the trophozoites 

 which multiply in this manner are termed schizonts ; and the minute 



