CHAPTER 11 



THE HYDRA, A SIMPLE MANY-CELLED ANIMAL 



In preceding chapters the structure and functions of the most 

 complex type of animal body were examined as a means of review- 

 ing and extending our knowledge concerning the body of man and 

 other animals. The organization and activities of the simplest 

 animals were Ukewise examined with a view to understanding the 

 resemblances between the highest and the lowest forms of animal 

 life. As the next step, one of the simplest many-celled animals 

 will be studied in order that we may determine the essentials of 

 the many-celled state. The Metazoa, or many-celled animals, 

 comprise the great bulk of the animal world, but there are many 

 kinds of Mctazoa. At the first survey there seems to be an end- 

 less diversity. When, however, one looks beneath the surface, 

 there is more unity; for the structural plans among the IMetazoa 

 are restricted in the manner described in the preceding chapter. 

 While the simplest type of metazoan is that represented by the 

 Mesozoa and by the Porifera, a more usable type for the present 

 purpose is that found in the Phylum Ccelenterata. Among coelen- 

 terates the fresh-water organism called hydra is a form that is 

 readily obtainable and one that is unusually well suited to illus- 

 trate the organization of a simple many-celled animal as well as 

 that of a coelenterate. 



The Hydra as a Simple Metazoan 



Occurrence and External Features of the Common Species of 

 Hydra. — Three species of the genus Hydra are commonly found 

 in the eastern and central portions of North America: H. viridis- 

 sima, which is green; H. vidgaris, w^hich is usually gray although 

 at times orange or brownish ; and //. oUgactis, which is brown and 

 has very long tentacles. The following account is sufficiently 

 generahzed to be applicable to any of these types unless otherwise 

 stated. The green hydras are smaller and more active, have 



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