294 Sporozoa 



polar cap over the micropyle; a thick brownish intermediate layer which 

 becomes quite thin at the micropyle; and a thin colorless endocyst. Al- 

 though there are a number of exceptions, size and structure of the oocyst 

 are often rather characteristic of the species. 



The zygote usually divides into sporoblasts, often leaving a residual 

 mass, and each sporoblast usually secretes a sporocyst membrane (Fig. 6. 

 14, B-E). Less commonly, sporocysts are not produced, sporozoites being 

 protected only by the oocyst membrane (Fig. 6. 14, F-H). The sporoblasts 

 of Karyolysus are unusual in that they are released from the ruptured 

 oocyst as motile elongated sporokinetes which invade the eggs of a mite. 

 The sporoblast then rounds up and secretes a sporocyst membrane. By 

 the time the egg has developed into a nymph, the sporoblast has produced 

 sporozoites. In the more typical Coccidia, both the number of sporocysts 

 and the number of sporozoites are features of taxonomic value. 



Sporogony may or may not be completed within the host. The oocysts 

 of avian and mammalian parasites are typically eliminated with the zy- 

 gote undivided or in the process of forming sporoblasts. Under favorable 

 conditions, sporozoites are developed within a few days, and the oocyst 

 is then infective for a new host. At the other extreme, the production of 

 sporozoites is completed within the host. Development of the latter type 

 has led to an interesting modification of the coccidian life-cycle in 

 Shellackia (Fig. 6. 19, A-F). Sporozoites are developed within an asporo- 

 cystic oocyst, which eventually ruptures in the intestinal connective tissue 

 of the reptilian host. The liberated sporozoites enter erythrocytes. In- 

 vaded blood corpuscles are swallowed by a mite, in which cells of the gut 

 wall apparently ingest the sporozoites without destroying them. If such a 

 mite is eaten by a vertebrate host the sporozoites are released and invade 

 the intestinal epithelium. A comparable transfer by leeches occurs in 

 Lankesterella, in which there is a similar invasion of erythrocytes by 

 sporozoites. 



In these cycles of Shellackia and Lankesterella the mite and the leech 

 are mechanical vectors in which the parasites do not undergo develop- 

 ment. A true intermediate host occurs in certain cases involving invasion 

 of erythrocytes or leucocytes by gametocytes. After ingestion of gameto- 

 cytes by a leech (Haemogregarina) or by a tick or mite {Hepatozoon, 

 Karyolysus), gametogenesis and syngamy occur and sporozoites are pro- 

 duced in the invertebrate. Transfer of sporozoites may be effected by 

 inoculation, as in the case of leeches feeding on a vertebrate, or by the 

 ingestion of infected mites. Aggregata eberthi (29) shows an unusual 

 cycle in which the "intermediate" host becomes infected by eating the 

 "final" host. Merogony occurs in the intestinal connective tissue of a crab 

 which has ingested sporocysts. If an infected crab is eaten by a squid, 

 some of the merozoites develop into gametocytes, and syngamy and 

 sporogony follow, 



