HYDRA 141 



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. 



167. 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 condition the green hydra has its 

 reactions somewhat modified, particularly its reaction to light, a liberal 

 supply of which is needed by the algal cells. 



168. 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 occurs, and very small 

 fragments may thus develop into complete animals. While regeneration 

 may result in an increase in numbers, it is not a normal method of 

 multiplication and cannot, therefore, be considered as reproduction. 

 A hydra which has been partially divided into parts may regenerate in 

 such a manner as to produce a compound animal with several hypo- 

 stomes, each with a mouth and a ring of tentacles. Parts of two indi- 

 viduals 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. 



