SYMBIOSIS 97 



The chief interest, biologically, of these green cells lies in their 

 physiological relations to Hydra. They are not parasites, 

 for parasites are organisms living at the expense of other 

 organisms to which they are detrimental, either structurally 

 or functionally. Hydra is not inconvenienced in any way by 

 the presence of these green cells, but lives, grows, and reproduces 

 normally. On the contrary there is . every reason to believe 

 that the foreign cells are beneficial to Hydra, for green plants 

 are essentially constructive in their vital activities and make 

 use of CO 2 for the construction of starch. This CO 2 is a waste 

 product of metabolism of the Hydra cells, and the green cells 

 are of service in freeing them of it; thus Hydra viridis will live 

 much longer in an atmosphere of CO 2 than Hydra fusca (Hadzi). 

 Furthermore the active green cells give off oxygen which the 

 Hydra cells need. Hydra viridis may be freed from its accom- 

 panying plant guests by being kept in dilute glycerine, and will 

 continue to live as a white Hydra although it does not have the 

 same vitality, and its growth and reproduction are much slower 

 (Whitney). It is probable, therefore, that the green cells are 

 beneficial in supplementing the normal metabolic processes of 

 Hydra. 



Just what advantages the green cells obtain from this 

 peculiar habitat are less obvious. They are protected by the 

 animal and probably receive a constant supply of CO 2 . Out 

 of the sunlight, as at night, they require oxygen and give off 

 CO 2 just as animal cells do; the oxygen at such times may be 

 obtained from the surrounding water by diffusion in the same 

 way that the Hydra cells obtain it, while the CO 2 and other 

 waste matters are probably disposed of in the same way as in 

 the case of Hydra fusca, by osmosis. On the whole, therefore, 

 there is some gain to both types of organisms by this joint house- 

 keeping, the advantages, which we can only guess at, being 

 proved by their thriving vitality. 



This phenomenon of living together is termed symbiosis 

 or messmateism, and differs from parasitism in the fact that no 

 structural or functional harm is done to either organism, host 

 and guests living together for mutual benefit. 



