HYDRA 



143 



Ecfoderm 



The same individual may produce both spermaries and ovaries at 

 the same time, in which case self-fertihzation is possible. They are 

 usually not so produced, however, and cross-fertilization is the rule. 



169. Symbiosis. — The green hydra, Hydra viridissima Pallas, exhibits 

 an interesting association between a plant and an animal. Each cell 

 of the hydra contains plant cells which are themselves individual one- 

 celled plants belonging to a group known as algae. These plant cells 

 possess chlorophyll and carry on photosynthesis. The association 

 therefore represents a partnership in which both partners profit, the 

 alga receiving carbon dioxide and nitrogen 

 from the hydra and the hydra in turn being 

 furnished with oxygen. Such an association 

 is called symbiosis. By virtue of this condi- 

 tion the green hydra has its reactions some- 

 what modified, particularly its reaction to 

 light, a liberal supply of which is needed by 

 the algal cells. 



170. Regeneration. — Regeneration is the 

 replacement by an animal of any portion of 

 the body which has been lost. It occurs 

 naturally after an accident has befallen the 

 individual, and it can be induced artificially 

 by mutilation. In the hydra it readily Entoderm 



occurs, and very small fragments may thus gree^'^hyd^TSX ''Ssi'^a! 

 develop into complete animals. While regen- showing the minute algal 

 eration may result in an increase in numbers, plants (represented by the 



■^ ^ ' larger black spots) withm the 



it is not a normal method of multiplication entoderm cells. (From WhU- 

 and cannot, therefore, be considered as repro- "^^' ^i^i^^sicai Bulletin, vol. 15.) 

 duction. A hydra which has been partially divided into parts may 

 regenerate in such a manner as to produce a compound animal with 

 several hypostomes, each with a mouth and a ring of tentacles. Parts 

 of two individuals may be grafted together, but they must be of the same 

 species. 



The hydra was the first animal known to have the power of regener- 

 ation, the discovery being made in 1744 by an Englishman named 

 Trembley. It has been a favorite type for experimentation in this 

 field ever since. 



