196 



THE PROTOZOA 



tiny organisms from the water as it passes over the gills. The 

 whale, the largest of all mammals, strains protozoans and other 

 small animals and plants out of the water by means of hanging 

 plates of whalebone, the slender filaments of which form a sieve 

 from the top to the bottom of the mouth. 



Relation of Protozoa to Disease. — The study of the life history 

 and habits of the Protozoa has resulted in the finding of many 

 parasitic forms, and the consequent explanation of some kinds 

 of disease. One parasitic protozoan like an amoeba is called 



Plasmodium malarice. It causes 

 the disease known as malaria. 

 Part of its life is passed with- 

 in the body of a mosquito (the 

 anopheles), into the stomach 

 of which it passes when the 

 mosquito sucks the blood from a 

 person having malaria. Within 

 the body of the mosquito a com- 

 plicated part of the life history 

 takes place, which results in a 

 stage of the parasite establish- 

 ing itself within the glands which 

 secrete the saliva of the mos- 

 quito. When the mosquito 

 pierces its human prey a second 

 time, some of the parasites are 

 introduced into the blood along with the saliva. These para- 

 sites enter the corpuscles of the blood, increase in size, and then 

 form spores. The rapid process of spore formation results in 

 the chill of malaria. Later, when the spores almost fill the blood 

 corpuscle, it bursts, and the parasites enter the fluid portion of the 

 blood. There they release a poison which causes the fever. The 

 spores may again enter the blood corpuscles and in forty-eight or 

 seventy-two hours repeat the process thus described. Yellow 

 fever is undoubtedly conveyed by another species of mosquito, 

 and is probably due to the presence of a protozoan similar to 

 that of malaria in the blood. That these diseases may be stamped 

 out by the destruction of the mosquitoes, by preventing their breed- 



Blood corpuscles of a patient with 

 malarial fever. Two corpuscles con- 

 tain the parasites. Photograph, 

 greatly enlarged, by Davison. 



