180 - The Cell 



A parasite may be neutral with reference 

 to one kind of host, but definitely patho- 

 genic to another. Such a case is provided by 

 Cryptocotyle, a parasitic flatworm. The usual 

 host of Cryptocotyle is a seagull, and adult 

 gulls are quite immune to the presence of 

 the parasite in their intestine. But mammals, 

 such as the dog and fox, may become seri- 

 ously ill when they are infected with Crypto- 

 cotyle. 



Natural immunity depends to a large ex- 

 tent upon the capacity of the host's tissues to 

 form antibody substances (pp. 335 and 336). 

 These antibodies counteract the toxic prod- 



ucts of the parasite; and the development of 

 natural immunity indicates that an associa- 

 tion between the parasite and host has ex- 

 isted for a comparatively long evolutionary 

 period. 



Nitrogen-fixing Bacteria. One case of sym- 

 biosis deserves special emphasis, because it 

 plays an important role in the conservation 

 of soil fertility. The nitrogen-fixing bacteria 

 live symbiotically in the root tissues of sev- 

 eral common species of higher plants (Fig. 

 10-6). These bacteria are the only organisms 

 that are able to utili/e free nitrogen (N 5 ) 

 in their metabolism. Such a usage involves 



Fig. 10-6. Nitrogen-fixing bacteria. These symbiotic organisms live in the cytoplasm of cells of nodules found 

 on the roots of clover and other leguminous plants. 



