THE CCELENTERATES AND SPONGES 



133 



lie at the bottom of a pond or stream. Enclosed in the 

 shell, the egg can withstand periods of drought and other 

 unfavorable conditions after which it may break out of 

 the shell and complete its development. 



Hydra has long been a classical object for the study 

 of regeneration. It may be cut in numerous pieces each 

 of which may form a new Hydra, and pieces from different 

 individuals may be grafted together, much as nurserymen 

 graft together different trees or shrubs. One species of 

 Hydra, Hydra viridis, is remarkable for its green color. 



FIG. 109. A, budding in Hydra; B, regeneration of Hydra from a small 

 piece from near the middle of the body. 



The green is due to the presence of small, unicellular green 

 plants (algae) in the cells of the entoderm. The plants 

 ordinarily are not digested, but live on material derived 

 from the host. As plants assimilate carbon dioxide which 

 is a waste product of animals and give off oxygen which 

 the animal uses in respiration, the association between the 

 Hydra and the algae is supposed to be to their mutual 

 advantage. Thus we have an illustration of symbiosis 

 which was briefly considered in a previous chapter. 



There are numerous marine relatives of Hydra which are 

 commonly called hydroids. Many of these are much 

 branched and form colonies. In some of these there has 



