MORPHOLOGY AND TAXONOMY OF THE SPOliOZOA 531 



There are thus two complete cycles in the life history of a typical 

 sporozoon, an asexual and a sexual cycle. There are many varia- 

 tions in different types and few life cycles conform exactly with 

 that of Eimeria. In the Eugregarines, for example, the asexual 

 cycle is entirely eliminated, the sporozoite developing directly into 

 a gametocyte. In Gregarines also we find a curious process which 

 recalls the phenomenon of conjugation in the Ciliata. It is termed 

 pseudo-conjugation. Two individuals come together side by side or 

 end to end and an envelope is secreted which encloses both indi- 



Fig. 213. — Lankesteria ascidiae. Young sporozoites enter epithelial cells (A, B, C) 

 and grow directly into gamonts (D) ; two of these unite in pseudo-conjugation (E), and 

 each forms gametes after repeated nuclear divisions (F, G, H). The gametes fuse 

 two and two (/, ./, A"), and the zygotes undergo three metagamic divisions, forming 

 eight sporozoites (L to O). The parent cells degenerate and the sporocysts are filled 

 with sporoblasts, each with eight sporozoites. (After Siedlecki.) 



viduals. This envelope is a gametocyst. Each individual now 

 forms a large number of gametes and those from one individual 

 fuse with the gametes from the other individual and a multitude 

 of zygotes is formed. The actual fertilization membrane becomes 

 the oocyst and sporocyst and the zygotes divide at once to form 

 sporozoites (Fig. 213). 



Invariably parasitic, there is the greatest diversity in sites of 

 parasitism and modes of life of Sporozoa. Gregarines are found only 

 in invertebrates while Coecidiomorpha and Cnidosporidia are not 



