342 ANIMAL BIOLOGY 



The life history of malarial parasites exhibits a continuous series 

 of adaptations to parasitic life: the nicety of the adjustment being 

 especially well illustrated at the transfer from Man to Mosquito, 

 since all the parasites which enter the stomach of the latter are 

 digested except those sexual forms which are ready to initiate the 

 sexual part of the cycle in the new host. (Figs. 246, 247.) 



But the acme of parasitic associations is only attained when 

 the adaptations of parasite and host have become so complete that 

 the latter ' pays the price ' without any ill effects. Thus the Ante- 

 lopes and similar Mammals of certain regions of Africa harbor 

 in their blood various species of Protozoan parasites, known as 

 Trypanosomes, without any apparent discomfort. But if the in- 

 termediate hosts, which are biting Flies, transfer certain species 

 of Trypanosomes to the blood of imported Horses or Cattle, or 

 of Man, serious diseases result which are usually fatal. Indeed, the 

 opening up of large regions of Africa has been greatly retarded 

 by the ravages of Trypanosomes in new hosts to which they are 

 not adapted. Generally speaking, pathogenic species may be re- 

 garded as aberrant forms which are not yet adapted to their hosts 

 or are not in their normal hosts. And these are the parasites which 

 are especially forced upon our attention, though there are few 

 organisms without their specially adapted parasites — the parasites 

 themselves not excepted. (Figs. 224, 245.) 



4. Immunity 



At best, however, the part played by the host cannot be re- 

 garded as ideal, and devious types of adaptations against parasites 

 exist which, in so far as they are effective, bring about immunity. 

 Usually among the higher animals, including Man, immunity 

 to the ravages of pathogenic microorganisms seems to depend 

 chiefly upon the activities of the white blood cells and upon spe- 

 cific chemical substances in the blood, termed antibodies. 



The white blood cells have been called the policemen of the 

 body because, under the influence of invading organisms, some 

 of them make their way through the walls of the capillaries in the 

 region of the infection and, in amoeboid fashion, engulf and digest 

 the intruders. When acting in this capacity they are referred to as 

 phagocytes. Similar phagocytic cells are found in groups, Peyer's 

 patches, scattered in the intestinal wall. Apparently they are 

 to forestall invasion of the tissues by Bacteria that swarm there. 



