I SPOROZOA 63 



trophic part of the life-history, i.e. through the period of active feeding, 

 or are concentrated into a special period at the end of the trophic stage. 

 These two main groups are known as the Neosporidia and the 

 Telosporidia. 



A. Telosporidia 



(i) Gregarinida. This group includes Monocystis. In it the tropho- 

 zoite is at first intracellular, living embedded in the protoplasm of the 

 host, but it is to be noted that the cell containing it is never a blood- 

 corpuscle. The parasite becomes free from the host-cell before sporogony 

 takes place and the number of sporozoites enclosed within one cyst or 

 capsule is usually eight. The gregarines occur as parasites in most of 

 the main groups of invertebrate animals except possibly the Mollusca. 



(2) CocciDiA. These occur as parasites in Arthropods {e.g. Centipedes), 

 in Molluscs (especially Gasteropods and Siphonopods) and in Vertebrates 

 {e.g. Rabbit). The trophozoite in this case remains throughout a more 

 or less spherical intracellular parasite, growing within the host-cell and 

 gradually destroying it. When fully grown it becomes a schizont and 

 divides into merozoites which infect new cells and in this way great 

 destruction of tissue may take place, resulting sometimes in the death 

 of the host. Even without this a limit is reached in the activity of the 

 schizogony process and sporogony takes place (also intracellular), gametes 

 being formed which conjugate to form zygotes. The spherical zygote 

 surrounds itself with a stout cyst which shelters it when it passes away 

 from the protection of the host's body. Within this cyst the zygote 

 divides into sporozoites the number of which differs in different members 

 of the group. The sporozoites are set free when the cyst is swallowed 

 by a suitable host and burrowing into host-cells start the life-cycle afresh 

 as young trophozoites. 



(3) Haemosporidia. In this group — exemplified by Plasmodium — 

 the trophozoite is for a time at least amoeboid and intracellular — the 

 host-cell being usually the red blood-corpuscle of a Vertebrate. Repro- 

 duction takes place by schizogony, followed after a time by sporogony — 

 the zygote giving rise to sporozoites which become free instead of remaining 

 shut up within a cyst. Typically the sporogony or sexual cycle is gone 

 through in the body of an intermediate host such as some species of 

 blood-sucking insect. 



Parasites closely allied to those which cause malaria in man occur 

 in various mammals and other vertebrates. In Birds there occur 

 commonly species of Plasmodium — sometimes separated off as a different 

 genus under the name Proteosoma — and Haemoproteus in which latter 



