i6o 



ELEMENTARY ZOOLOGY. 



cells divide into many of these rounded bodies (spores) when they 

 reproduce. These animals are all parasites on other animals, and 

 are usually even simpler in structure than the types we have been 

 studying. Their life history (as is the case with most parasites) 

 is quite complex. They often pass through a number of stages 

 some active in which they move about in the body fluids of the 

 host, some encysted in which they divide rapidly and produce many 

 spores, which become motile again and attack new cells. 



In the case of the parasite which produces malaria in man (Plasmo- 

 dium malaria;) , it works its way into the red cells of the blood. Here 

 it multiples and at the same time manufactures certain poisons 

 which produce fever in the host. These new spores escape into the 



FIG. 41. Diagram showing the resting position of the body of the Anopheles, or 

 malaria-carrying mosquito (A) ; and of Culex, another common mosquito (B). Adapted 

 from Waterhouse. 



blood, enter new red cells, until very great and dangerous infection 

 may take place. If a mosquito "bites" a person with malaria, 

 some of these cells are drawn up into the stomach of the mosquito. 

 In the mosquito two of the cells unite or conjugate, and after some 

 changes get into the salivary glands of the animal and from this 

 point they pass to the blood of another human victim when this 

 mosquito again seeks its food. (See Fig. 7, 5.) 



A similar protozoan, also carried by a species of mosquito, is 

 responsible for the disease known as yellow fever. A number of 

 diseases of animals are produced by these sporozoa. They are 

 therefore very important to man. The discovery of their work is 

 very recent. The knowledge of the part that mcsquitoes play in 



