SYMBIOSIS, COMMENSALISM, PARASITISM 193 



In a similar way, thousands of species of animals become 

 adapted to a parasitic life in different types of host. Further- 

 more, there are different grades of parasitism; some are obliga- 

 tory parasites, requiring a particular host and a particular organ, 

 very often, of that host. Others are facultative parasites, not 

 absolutely dependent on a given host, but capable of living in 

 such a host if chance brings them there. Thus the round worm, 

 Trichina, is an obligatory parasite of man, and a facultative 

 parasite of domesticated animals, including the pig. The 

 embryos are eaten with infected meat; liberated in the human 

 intestine, they penetrate the walls of the digestive tract and 

 multiply in the body cavity, ultimately penetrating muscle 

 bundles where, in the muscle cells, they finally encyst. If the 

 unfortunate victim does not die from trichinosis before such 

 encystment occurs, recovery is possible, for once encysted, 

 the parasites do no further damage. Trichinosis, however, is 

 usually fatal, the infected muscles of the victim often containing 

 millions of the parasites (Fig. 85). 



B. ANIMAL ASSOCIATIONS 



Animals of different kinds may live together in harmony and 

 without ill effects on either; or different types of living organ- 

 isms may live together for mutual benefit. Such a form of 

 partnership is called symbiosis, an example of which we have 

 seen in the case of Hydra viridis or this association may become 

 obligatory, so that neither organism can live without the other. 

 Where the association does not confer mutual benefit, or any 

 obvious advantage to both, we speak of the association as com- 

 mensalism. A good example of this is the union of the glass 

 sponge, Euplectella, and a crustacean; a pair, male and fe- 

 male, enter the pores of the sponge in the larval stage, and 

 grow to adult size within the chosen prison, which they cannot 

 leave after they grow up (Fig. 86). Numerous examples 

 may be found, in the human intestine, of both commensalism 

 and symbiosis; many innocuous protozoa and bacteria live there, 

 while many bacteria also are symbionts which play an important 



