SOCIAL LIFE OF ANIMALS 483 



which are ciliated Protozoa inhabiting their digestive tracts (p. 30). 



An example of animal-animal symbiosis is the case of termites 

 which live in beneficial relationship with the flagellates, Tricho- 

 nympha and allied genera (p. 29). 



Parasitism. (Gr. para, beside; sitos, food.) — In Biology, 

 parasitism is the condition of an organism which obtains its nourish- 

 ment wholly or in part from the body of another living organism 

 and which usually brings about some modifications in both guest 

 and host. In true parasitism, the bodies of the host and the parasite 

 must be in temporary or permanent contact other than that in- 

 volved in preying and capturing. The presence of the parasite 

 must not be beneficial. Ectoparasites may be temporary such as 

 plant lice, caterpillars, mosquitoes, flies and the leech; or permanent, 

 such as bird lice (for the bird), body lice and chigoes (chiggers). 

 Endo-parasites include intestinal worms like the tapeworm, hook- 

 worm, and pin- worm; and blood parasites such as the malarial 

 sporozoan and the filarian worms. 



The Effects of Parasitism. — i. On the Parasite. — Any parasite 

 may undergo some degeneration. In many cases, little remains 

 but the reproductive apparatus. 



1. On the Host. — The host may be very resistant, and by gather- 

 ing extra food render the parasite harmless; many white blood 

 corpuscles may be assembled and carry away the organisms. The 

 host may enclose the parasite in a cyst; it may develop counter 

 poisons, killing the parasite; or anti-bodies may neutralize the 

 injurious substance formed by the parasite. The host, for some 

 time apparently uninjured, may have its resistance so lowered by 

 the rapid multiplication of the parasites that it will succumb to 

 what ordinarily might be a minor ailment or injury.^ 



Association of the Same Species 



Colonies. — (i) Certain hydroids have a diverse differentiation 

 of the individuals. Coelenterate polyps are modified for feeding, 

 reproduction and locomotion. (2) The " Portuguese Man of War " 

 has specialized structures for feeding, reproduction and protective 

 and aggressive activities. (3) Sponges have cellular differentiation 

 for protection, ingestion, storage, distribution of food and reproduc- 



^ See Stunkard, H. W. 1929. Parasitism as a biological phenomenon. Sc. 

 Men., vol. 28, pp. 349-362, April. 



