HYDRA 63 



The interesting fact is that, although these plant cells are actu- 

 ally living in the cells of Hydra, they do not cause any harm to 

 their host. On the contrary it appears that there is a mutual bene- 

 fit derived from the partnership. A condition of this kind in which 

 both organisms derive benefit from the association is known as 

 symbiosis, and is to be distinguished from parasitism, in which 

 one organism, the parasite, lives at the expense of another or- 

 ganism, the host. The benefit to the plant arises from the fact 

 that some of the metabolic wastes of the Hydra are just what are 

 needed for photosynthesis. On the other hand, as a result of the 

 photosynthesis, an excess of oxygen is liberated, and this the Hydra 

 secures for its own life processes. It is also probable that the 

 Hydra makes use of any surplus food which may be manufactured 

 by the plant cells. At any rate, experimental work has shown that, 

 by placing green Hydras in certain environments it is possible to 

 stop the food formation : the animals under such conditions be- 

 come white, and, although they are able to survive, they do not 

 show so great vitality as do other individuals in which the 

 symbiotic condition is maintained. 



2. Respiration and Excretion 



There are no specialized organs of excretion in Hydra. Each of 

 the cells apparently attends to the excretion of its own metabolic 

 wastes just as the protozoan cells do. Respiration is carried on 

 through the permeable cell walls and takes place both at the 

 surfaces of the ectoderm cells, which are in contact with the sur- 

 rounding water, and also at the surfaces of endoderm cells, which 

 are in contact with the fluids of the enteric cavity. 



3. Reproduction 



Reproduction in Hydra may take place either asexually or sexu- 

 ally. The common asexual method is known as budding, and, as 

 in Grantia, may often be observed in actively growing individ- 

 uals. Certain cells in the body wall, generally near the basal disc, 

 begin to divide rapidly and soon form a small swelling, which 

 projects from the ectoderm. This projection continues to increase 

 in size and soon shows a differentiation into the body proper and 

 into the tentacles which surround a central mouth opening, and 

 soon the bud resembles a small, attached Hydra. During the 

 formation of the bud, both the body wall and enteric cavity of 



