220 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



Keeping the Cycle Going 



The chief problem of any species centers about maintaining itself, 

 a statement to which there is no exception in the world of parasites. 

 Obviously those organisms which have become adapted as ectopara- 

 sites are not faced with complicated problems relating to the transfer 

 from host to host. By means of simple contact a new host may be 

 reached, or if a portion of the life cycle of the parasite is free living, 

 it may leave the host to deposit its eggs. Even in cases where the 

 eggs are laid among hairs of the host they usually fall to the ground 

 to develop. 



A more difficult problem of maintaining the species must be faced 

 when internal parasites are involved. Bacteria which are capable of 

 producing protective capsules or spores of one sort or another are 

 tided over unfavorable periods and so aided in reaching new hosts. 

 They are adapted also for rapid reproduction. One worker has esti- 

 mated that if the multiplication of bacteria were unchecked one cell 

 would be the parent of 281,500,000,000 bacteria in two days. Such a 

 mass at the end of the third day would weigh about 148,356,000 

 pounds. 



Many parasitic protozoa as well as metazoa are adapted to be 

 transferred from one host to the next by means of resistant cysts 

 secreted by the organism. Others, like the blood-inhabiting try- 

 panosomes or malarial organisms, secure transference by adapting 

 themselves to insects which act as wholesale distributors for the 

 parasites. Some produce harmful toxins which occasionally kill the 

 host. In such instances, however, one may be sure that the host is 

 abnormal and the parasites have not become adapted to it. In the 

 case of the trypanosomes of man in Africa, antelopes are their natural 

 hosts and are quite tolerant to these blood parasites. Since man and 

 domestic animals are unnatural hosts, they are consequently much 

 more severely affected by them. 



The Complexity of Parasitic Relationships 



The most satisfactory way to secure a general idea of the surpris- 

 ingly varied adaptations to a parasitic existence is by a study of 

 a few examples. Such a study emphasizes clearly the almost uncanny 

 adaptations which have been made by parasites to insure the com- 

 pletion of their life cycles. While various types of parasitism clearly 

 exist, nevertheless the line that demarks one kind of parasite from 



