Il8 COLLEGE ZOOLOGY 



Hydra, but in many other coelenterates, and in some of the rep- 

 resentatives of almost every phylum of the animal kingdom. 

 Hydra, however, is a species that has been quite widely used 

 for experimentation. Pieces of Hydra that measure \ mm. or 

 more in diameter are capable of becoming entire animals (Fig. 

 72, B). The tissues in some cases restore the lost parts by a mul- 

 tiplication of their cells; in other cases, they are worked over 

 directly into a new but smaller individual. Parts of one Hydra 

 may easily be grafted upon another (Fig. 72, C). In this way 

 many bizarre effects have been produced. 



Space will not permit a detailed account of the many interesting 

 questions involved in the phenomenon of regeneration, but enough 

 has been given to indicate the nature of the process. The benefit 

 to the animal of the ability to regenerate lost parts is obvious. 

 Such an animal, in many cases, will succeed in the struggle for 

 existence under adverse conditions, since it is able to regain its 

 normal condition even after severe injuries. Physiological re- 

 generation takes place continually in all animals; for example, 

 new cells are produced in the epidermis of man to take the place 

 of those that are no longer able to perform their proper functions. 



Both internal and external factors have an influence upon the 

 rate of regeneration and upon the character of the new part. 

 Temperature, food, light, gravity, and contact are some of the 

 external factors. In man, various tissues are capable of regen- 

 eration ; for example, the skin, muscles, nerves, blood-vessels, and 

 bones. Lost parts are not restored in man because the growing 

 tissues do not coordinate properly. Many theories have been 

 advanced to explain regenerative processes, but none has gained 

 sufficient acceptance to warrant its inclusion here. 



2. Class I. Hydrozoa 



Hydra is the Hydrozoon which is most easily obtained for 

 study, and by means of Hydra the principal characteristics of the 

 coelenterates have been illustrated. There are, however, a vast 

 number of related animals that differ widely in form, structure, 



