ii PHYLUM PROTOZOA 81 



moving to and fro actively for a time, unite in pairs after the 

 substance of the two individuals has become coalescent (D). From 

 each of the cells or zygotes that are formed by the union of two 

 of the original small cells or gametes, a spore is formed, so that the 

 cyst now comes to contain numerous small spores (E). These are 

 spindle-shaped bodies, each enclosed in a strong chitinoid case (F), 

 and thus differing in a marked manner from the naked spores 

 of the Rhizopoda and Mastigophora. The protoplasm and nucleus 

 of each spore then undergo fission, becoming divided into a number 

 of somewhat sickle-shaped bodies which are arranged within the 

 spore-coat somewhat like a bundle of sausages. By the rupture 

 of the spore-coat these falciform young or sporozoitcs are liberated, 

 and at once begin active movements, the thin end of the body 

 moving to and fro like a clumsy flagellum. The falciform young 

 appear, in fact, to be greatly modified flagellulae. They make their 

 way to the clumps of developing sperms, bore their way in, and 

 are thus found surrounded by sperm-cells in various stages of 

 development (G). After thus living an intracellular life for 

 a time, they escape (H) into the cavity of the vesicula and grow 

 into the adult form. 



2. Classification and General Organisation. 



The Sporozoa are exclusively parasitic, being the only group of 

 Protozoa of which this can be said. They have no organs of 

 locomotion and always multiply by spore -formation. The class is 

 divisible into the following five orders : — 



Order 1. — Gregarinida. 

 Sporozoa in which the trophozoite is free and motile. 



Order 2.— Coccidiidea. 



Sporozoa in which the trophozoite is a minute intracellular 

 parasite. 



Order 3. — H^mosporidia. 



Sporozoa in which the trophozoite is amoeboid, and lives as 

 a parasite in the coloured blood-corpuscles of Vertebrates. 



Order 4. — Myxosporidea. 



Sporozoa in which the trophozoite is amoeboid, but not intra- 

 cellular. 



Order 5. — Sarcocystidea. 



Elongated Sporozoa, usually found in muscle. 

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