Classes of protozoa 129 



parasite now enters upon a spore-forming stage. Certain 

 merozoites grow more slowly than the others, and instead of 

 becoming schizonts give rise to elements of two types, viz. 

 microgametes, slender cells bearing a flagellum at each end, 

 which are male, and macrogametes, larger bean-shaped 

 cells, which are female. The latter after maturation free 

 themselves from the host cell, and in the cavity of the gut 

 are fertilised by a male element. After fertilisation, a trans- 

 parent membrane forms around the zygote (fertilised cell). 

 This membrane in the first instance serves to exclude all 

 microgametes after the first, and later, becoming very tough 

 and resistant, forms a protecting envelope or oocyst. After 

 the oocyst is formed the parasite may pass from the host to 

 the exterior or remain for some time longer within it. The 

 nucleus of the zygote within the oocyst now divides into 

 four, around which the protoplasm aggregates itself to form 

 the spores. There are thus four spores within a cyst. 

 Each spore divides, forming two sporo.zottes , which on the 

 arrival of the oocyst in the gut of a fresh host are liberated, 

 and attacking the lining epithelium recommence the life- 

 history. 



General Classification of Protozoa 



Since the Protozoa are unicellular organisms (except the 

 few which form loose colonies), their classification should 

 be harmonious with that of the cells in a higher animal. 

 This is so. Thus (a) the Rhizopods, in which the living 

 matter flows out in changeful threads or " pseudopodia," as 

 in the common Amoeba^ are comparable with the white 

 blood corpuscles or leucocytes, many young ova, and other 

 " amoeboid " cells of higher animals ; (b) the Infusorians, 

 which have a definite rind and bear motile lashes (cilia 

 or flagella), e.g. the common Paramoecium^ may be likened 

 to the cells of ciliated epithelium, or to the active sperma- 

 tozoa of higher animals ; {c) the parasitic Sporozoa, which 

 have a rind and no motile processes or outflowings, may 

 be compared to degenerate muscle cells, or to mature ova, 

 or to " encysted " passive cells in higher animals. 



This comparison has been worked out by Professor Geddes, who also 

 points out that the classification represents the three physiological 



