ORGANIC RESPONSE 283 



of the species which attaches itself to it. Such a 

 condition is called parasitism, and it is very wide- 

 spread throughout the animal kingdom. Rather 

 arbitrarily, zoologists discriminate between parasites 

 that attach themselves to the outside of their host 

 (ectoparasites), and those that dwell within the body 

 of the host (endoparasites) . , 



Parasitism in Protozoa. A large and important 

 group of the Protozoa (the Sporozoa) has adopted 

 an exclusively parasitic mode of existence. The 

 most familiar as well as the most important example 

 is the Plasmodium malaria, which is parasitic in 

 the blood of man and also in a certain species of 

 mosquito (Anopheles maculipennis) . This intro- 

 duces us at once to a remarkable feature of para- 

 sitism which we find in nearly all the groups, the 

 alternation of two very different hosts in which por- 

 tions of the life-cycle of the parasite are spent. 



The malaria-producing parasite lives in the red 

 blood-cells of man (an allied form is found also in 

 birds). It reproduces rapidly by multiple fission 

 (sporulation), each new individual attacking a 

 corpuscle, until, in extreme cases, nearly every 

 blood -cell in the victim is parasitized. The charac- 

 teristic fever of malaria is apparently due to the 

 liberation of some toxin produced by the parasites 

 simultaneously with their sporulation. In addition 

 to the asexual reproduction just described there are 

 also produced gametes, of two sizes, analogous to 

 sperm and ova. The vegetative phase has some- 



