COCCIDIOMORPHA 85 



The gametes leave the host cell and unite while free in the gut 

 cavity. The zygote nucleus undergoes what is probably a reduc- 

 tion division and encysts. Within its cyst (the oocyst) it divides 

 by multiple fission into four sporoblasts each of which forms a 

 cyst of its own (a secondary sporocyst) in which it divides into two 

 sporozoites. Thus sporogony takes place in two stages. In each of 

 these there is some residual protoplasm. Meanwhile the oocyst has 

 passed out of the host in the faeces. Infection of a new host takes 

 place by contamination of food by the encysted spores, which hatch 

 in the intestine. 



Aggregata is remarkable among coccidians for having two hosts. 

 Its agamogony takes place in crabs and involves a generation of 

 sporoblasts, but is not repeated. A cuttlefish, devouring a crab, 

 ingests the agametes, which in the new host proceed to become 

 gamonts. After gamogony with flagellate male gametes, fertilization, 

 and sporogony, the spores, containing four or more sporozoites, are 

 passed with the faeces of the mollusc and swallowed by a crab. 



Adelea is parasitic in the epithelium of the gut of Lithobius. Its life 

 history resembles that of Eimeria, but the gamonts, which difl^er con- 

 siderably in size, the male being the smaller, become free and form 

 a syzygy in the gut, though without encystment. The male gametes 

 are consequently not under the necessity of reaching the female by 

 swimming, and are not flagellated. 



Haemogregarina has become completely a blood parasite, and has 

 a life history closely resembling that of the Haemosporidia, with the 

 sexual process in an invertebrate host (see below). Since, however, it 

 undergoes syzygy, the organism would appear to belong to the Adelea 

 stock, whereas the Haemosporidia are probably related to Eimeria. 



Schellackia and Lankesterella, which have no syzygy, are transitional 

 to the Haemosporidia, under which (on p. 86) their life histories 

 are described. 



Suborder HAEMOSPORIDIA 



Coccidiomorpha, always true blood parasites; which have naked 

 sporozoites ; a locomotory zygote (ookinete) ; and no syzygy. 



The members of this suborder are intracellular blood parasites of 

 vertebrates and contain granules of pigment (melanin) derived from 

 the haemoglobin of the host — a feature which is lacking in the blood 

 parasites that belong to the Coccidia. They are transmitted from one 

 vertebrate host to the next by a blood-sucking invertebrate. Their 

 agamogony and the formation of their gamonts take place in blood 

 cells of the vertebrate host, but their gametes are formed, and ferti- 

 lization takes place, in the invertebrate. A series of intermediate cases 

 shows how this condition may have arisen. 



