50 COLLEGE ZOOLOGY 



eight elongated bodies called sporozoites (K, A, spz). Each 

 sporozoite penetrates a bundle of sperm mother cells (B, sp) of 

 the earthworm, and is then termed a trophozoite (B, tr). Here it 

 lives at the expense of the cells among which it lies. The 

 spermatozoa of the earthworm, which are deprived of nourish- 

 ment by the parasite, slowly shrivel up (C), finally becoming 

 tiny filaments on the surface of the trophozoite (D). 



When this stage is reached, two trophozoites come together (E) 

 and are surrounded by a common two-layered cyst wall (F, ep, 

 en). Each then divides, producing a number of small cells called 

 gametes (G). The gametes unite in pairs (H) to form zygotes 

 (zy). It is probable that the gametes produced by one of the 

 trophozoites do not fuse with each other, but with gametes 

 produced by the other trophozoite enclosed in the cyst. Each 

 zygote becomes lemon-shaped, and secretes a thin hard wall about 

 itself. It is now known as a sporoblast (/). The nucleus of the 

 sporoblast divides successively into two, four, and finally eight 

 daughter nuclei (/); each of these, together with a portion of 

 the cytoplasm, becomes a sporozoite (K, A, spz). 



b. Plasmodium mvax 



One of the best known of all the SPOROZOA is Plasmodium 

 vivax, which causes malarial fever. This minute animal was 

 discovered in the blood of malaria patients by a French military 

 doctor, Laveran. It was suggested by this investigator, in 

 1891, that the parasite is probably transmitted from man to man 

 by some blood-sucking insects, and this hypothesis was proved 

 to be correct by the work of Major Ross in 1899. Not only was 

 it demonstrated that malaria is spread by insects, but it was 

 proved that human beings can only become infected by the bite 

 of a diseased mosquito belonging to the genus Anopheles. The 

 two most common genera of mosquitoes are Culex and Ano- 

 pheles. One of the easiest methods of distinguishing one from 

 the other is by observing their position when at rest. It will be 

 found that the harmless Culex holds its abdomen approximately 



