270 Sporozoa 



/jLLL KNOWN Sporozoa are parasitic. The usual infective stage is 

 a sporozoite (Telosporidea), or an analogous sporoplasm (Cnidosporidea) 

 which may be ingested by a new host or inoculated by some vector. 

 Except in sjoecies transferred by inoculation, sporozoites are typically en- 

 closed within a spore membrane, the origin of which varies in different 

 groups. Sporozoa are not ciliated, and flagella are limited to the micro- 

 gametes of certain species. Nutrition is predominantly saprozoic, although 

 trophozoites of Nosema miitabilis apparently can ingest solid particles 

 (76). 



The group as usually defined shows a lack of homogeneity which led 

 Wenyon (139) to restrict his Class Sporozoa to the Gregarinidia, Coccidia, 

 and Haemosporidia — the Telosporidea as listed beloAv — and to recognize 

 the Cnidosporidea as a group of equal taxonomic rank. This arrangement 

 expresses clearly the general belief that Sporozoa are not monophyletic in 

 origin. However, the more common usage will be followed here, dividing 

 the group into three classes, Telosporidea, Cnidosporidea, and Acnido- 

 sporidea. 



CLASS 1. TELOSPORIDEA 



The life-cycle typically shows asexual and sexual phases, both of 

 which, except in the Eugregarinida, are characterized by reproduction. 

 Reproduction in the sexual phase produces sporozoites, either directly 

 from the zygote or from intermediate sporoblasts arising by division of 

 the zygote. In such forms as malarial parasites sporozoites are clearly the 

 result of schizogony (or sporogony, in this phase), but the appropriateness 

 of this term is less obvious in certain TelosjDoridea which produce two 

 sporozoites from each sporoblast. Sporozoites may be naked, or they may 

 be produced within a spore membrane. The membrane, which often 

 consists of inore than one layer, may be secreted by th^ zygote, as in typical 

 gregarines; or an encysted zygote may divide into two or more sporoblasts, 

 each of which secretes a spore membrane. In the first case, the spore de- 

 velops from an oocyst (encysted zygote); in the latter, from sporocysts 

 (encysted sporoblasts). With a few possible exceptions, the membrane ap- 

 parently is not divided into valves. Furthermore, there are no polar cap- 

 sules in spores of the Telosporidea. 



The asexual phase of the cycle is initiated by a sporozoite upon reach- 

 ing a host. Growth of the sporozoite into a mature trophozoite (schizont) 

 is followed by schizogony (or merogony, in this phase), except in the 

 Eugregarinida. The trophozoite remains uninucleate throughout much of 

 the growth period so that a plasmodium is usually developed shortly be- 

 fore merogony. Many Telosporidea are intracellular during this asexual 

 phase. However, some of the gregarines are intracellular only in the early 



