Sporozoa 311 



infants may show no history of previous toxoplasmosis. Human infections 

 have been diagnosed microscopically and by inoculation of laboratory 

 animals. A complement-fixation test also shows some promise for diagnosis 

 of active toxoplasmosis (136). 



CLASS 2. CNIDOSPORIDEA 



A general characteristic of this group is the production of spores 

 which differ distinctly from those of Telosporidea. Each spore (Fig. 6. 

 26, A) typically contains one or more polar capsules and also one or more 

 sporoplasms analogous to the sporozoites of Telosporidea. Each polar 

 capsule contains a coiled polar filament (Fig. 6. 26, C) which is extruded 

 under certain conditions. This filament has been considered an organelle 

 of attachment which prevents passage of the spore through the gut of 

 the host before the sporoplasm can emerge. Another view is that the 

 polar filament is a tube through which the sporoplasm travels from the 

 spore directly into a tissue cell (104). The membrane of the spore may 

 be apparently continuous, or it may consist of two or three sections, or 

 valves (Fig. 6. 26, B, E). In many Cnidosporidea each spore appears to 

 be multicellular in origin, in contrast to the sporocysts and oocysts of 

 Telosporidea and the cysts of other Protozoa. Another distinction between 

 the two classes is that the zygote of the Telosporidea undergoes sporogony, 

 while that of the Cnidosporidea gives rise to one or more trophozoites. 

 The young trophozoite is a small amoeboid organism which typically 

 develops into a plasmodium (Fig. 6. 26, D, F). However, the trophozoites 

 of Microsporida, which are almost exclusively intracellular parasites, are 

 very small and the nuclei are few in number. The trophozoites of other 

 Cnidosporidea typically invade body cavities of the host or else grow as 

 tissue parasites (intercellular rather than intracellular). Reproduction of 

 the trophozoite — by a so-called schizogony in certain Microsporida, or 

 by fission, budding, or plasmotomy in other cases — has been reported. 

 However, this phase of the cycle seems to have been eliminated com- 

 pletely in many Cnidosporidea. 



The Cnidosporidea have been divided into four orders: Myxosporida, 

 Microsporida, Actinomyxida, and Helicosporida. The spores of Myxo- 

 sporida are bivalve, usually with two, but sometimes one or four polar 

 capsules. The spore of the Actinomyxida contains three valves, three 

 polar capsules, and one to many sporoplasms. The spores of Microsporida 

 are small, usually with only one polar capsule, and the presence of 

 separate valves is doubtful in most species. The spore of Helicosporida 

 contains a single coiled filament but no polar capsules. 



Order 1. Myxosporida. The Myxosporida are mostly parasites of fishes, 

 less commonly of Amphibia and Reptilia. The supposedly more primitive 

 types are coelozoic, invading the gall-bladder, kidney tubules, and urinary 

 bladder. Others have been found in most tissues and organs of fishes. 



