THE ANIMAL AND ITS ENVIRONMENT 



eododern? ectoderno Rrrrrrrr- 



97 



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•• M : - ,, 



wood pdrTicles 



termite 



Fig. 5-9. Two examples of mutualism: the hydra with its algae, and the termite with its intestinal Protozoa. Note 



that the first instance is between a plant and an animal. 



vive indefinitely on pure carbohydrate. 

 They feed on wood alone, never receiving 

 any observable nitrogen to build proto- 

 plasm. Upon investigation it was found that 

 great hordes of complex Protozoa inhabit 

 the termite's intestines. If the termite was 

 warmed up a bit the Protozoa died and 

 such defaunated termites lived only a short 

 time. Likewise, the Protozoa could not sur- 

 vive outside the body of the termite. Ap- 

 parently the Protozoa in some way provide 

 the nitrogen essential for the termite. The 

 termite, on the other hand, provides a good 

 abundant home for the Protozoa. 



Parasitism. This is also a forced relation- 

 ship between two animals, but it is a one- 



way proposition. The parasite lives at the 

 expense of the host, taking all and giving 

 nothing in return, possibly even causing 

 injury to the host. An ideal parasite with- 

 draws just enough nourishment from its 

 host to maintain itself in good health and 

 in reasonable numbers. If the parasite re- 

 moves too much from its host, so that the 

 latter becomes sick and dies, then the para- 

 site too is destroyed. Many parasites reach 

 a satisfactory balance with their host in 

 which the latter merely contributes a home 

 for the parasite and is not apparently in- 

 jured by it. 



Parasitism should be distinguished from 

 predation, in which one animal also lives 



