524 



GENERAL CONSIDERATIONS 



566. Symbiosis.— If symbiosis is to be clearly separated from mutu- 

 alism, the separation must be on the basis of maximum intimacy and 

 the vital nature of the association. It is essentially an extreme form 

 of mutuahsm. One case of symbiosis has been noted in the relationship 

 which exists between a hydra and a green alga (Sec. 169), the cells of the 

 alga living a symbiotic life in those of the animal organism and furnishing 

 oxygen to the hydra in return for its own food. Other similar cases 

 are known. Symbiosis is also shown by the termite and the protozoan 

 symbiont which lives in its intestine (Sec. 319). It was long a matter of 

 speculation as to how termites are able to digest the cellulose of the wood 



Fig. 365.— Hermit crab in a snail shell, which also bears two sea anemones. Con- 

 sidered by some as an illustration of commensalism, but referred to here as one of mutualism. 

 From a preserved specimen. X %. 



on which they feed, since other insects are not known to have this ability, 

 Cleveland has recently discovered that this is due to the presence in the 

 intestine of the termite of a protozoan which prepares the wood for 

 digestion and absorption by the insect. In the absence of the protozoan 

 the termite is unable to use this food. On the other hand, the protozoan 

 finds appropriate conditions for existence only in the intestine of the 

 termite, and thus the association is vital to both. 



567. Parasitism.— The associations of organisms of different species 

 which have so far been defined all involve benefit to one or both but 

 injury to neither. If injury is done to one, then the association becomes 

 either one of parasitism or predatism. 



Parasitism has already been defined as the association of two organisms 

 of different species in which one, termed the parasite, lives at the expense 



