378 GENERAL ZOOLOGY 



a large number of zygotes. Each zygote divides to form a large 

 number of sporozoites, which penetrate the epithelial cells of the 

 host's intestine and grow into trophozoites. Only a single 

 host is necessary for the completion of the life cycle. The 

 trophozoite eventually emerges from the epithelial cells but 

 remains attached for a time by the epimerite. The latter 

 disappears when the parasite becomes free in the intestine. 

 Order 2. Coccidiomorpha. Parasitic in many invertebrates and 

 vertebrates. 



Examples: Coccidium schubergi, an intestinal parasite of the 

 centipede (Fig. 211); Plasmodium vivax, a blood parasite caus- 

 ing tertian malaria in man. The sexual phase of the life cycle 

 is passed in the body of the female of a species of mosquito 

 belonging to the genus Anopheles. An asexual phase, includ- 

 ing preparatory steps for the sexual phase, is passed in the 

 human blood stream (Fig. 212). 



The salivary glands of a mosquito capable of infection carry 

 sporozoites, which are spindle-shaped cells, 10 to 12 n in length. 

 These are introduced into the wound when the mosquito 

 bites a human being. In the human blood stream the parasite 

 bores into a red blood corpuscle, where it takes on an amoeboid 

 shape known as the trophozoite stage. On reaching its full 

 growth, the trophozoite undergoes segmentation or schizogony, 

 as it is called, to form spores or merozoites, which are liberated 

 in the blood stream by the rupture of the corpuscle, about 48 

 hours after infection. The free merozoites attack fresh cor- 

 puscles and the cycle is repeated; each time the corpuscles 

 break down, the chill characteristic of malaria occurs. After 

 several generations of merozoites have been produced, two 

 kinds of gametocytes are formed, macrogametocytes and 

 microgametocytes. The factors determining the development 

 of the merozoites into gametocytes instead of trophozoites are 

 unknown. For further development and complete differentia- 

 tion, the gametocytes must pass into the stomach of the 

 mosquito, a transfer readily brought about when a mosquito 

 bites a patient containing them. The macrogametocyte 

 undergoes certain nuclear changes somewhat akin to polar- 

 body formation and is thus transformed into a macrogamete. 

 Each microgametocyte, on the other hand, produces from six 

 to eight whiplike microgametes. A single microgamete enters 



