102 PHYLUM PROTOZOA- -THE SIMPLEST ANIMALS. 



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and the parasite now enters the schizont stage, where its 

 nucleus divides into a number of daughter nuclei. These 

 arrange themselves around the periphery of the cell, whilst 

 the protoplasm breaks up to form along with them bodies of 

 a shape similar to the sporozoites. There are important 

 structural differences, however, apart from the difference in 

 origin. The parasites, now known as merozoites, rupture 

 the host cell, move in the gut cavity after the manner of the 

 sporozoites, enter fresh epithelial cells, and repeat the fore- 

 going cycle until ultimately the greater part of the gut 

 epithelium is destroyed. In about five days, however, 

 owing perhaps to the failing capacity of the host to nourish, 

 the limit of asexual reproductivity is reached, and the 

 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 them- 

 selves 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 sporozoites, 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 



