IX 



DIVISION OF LABOR IN THE COELENTERATES 



Preview. The Hydra, a representative of the phylum Coelcnterata ; the 

 ectoderm and its functions ; the endoderm and its functions ; reactions to 

 stimuli ; reproduction ; regeneration ■ Hydroids • Suggested readings. 



PREVIEW 



It has already been shown that unicellular animals may exhibit 

 considerable complexity of structure, and that associated with this 

 complexity, there is a separation of functions in different parts of the 

 cell, but we have not traced this division of labor into the many- 

 celled animals or metazoa. The colonial forms, such as Pandorina, 

 Eudorina, and Volvox, claimed by both botanists and zoologists, are 

 interesting exam]iles of aggregations of many cells showing little 

 evidence of organization or division of labor. Even in the colony of 

 Volvox, most of the cells have common functions, only the reproduc- 

 tive cells being set off from the others. 



The Hydra, a tiny animal little higher in the scale of life, gives every 

 evidence in its structure of being a simple organism and not just a 

 collection, or colony, of cells. It shows, in a convincing manner, how 

 a simple, many-celled organism lives. It answers the question of 

 how division of labor might arise among the cells of a simple organism, 

 For this reason it is chosen as a type in most courses in biology and 

 so has a place in this text. 



The Hydra, a Representative of the Phylum Coelenterata 



Hydras are quite abundant in many ponds or slow-moving streams, 

 where they may be collected on the stems and leaves of aquatic plants. 

 In an aquarium, they often leave these plants and become attached 

 to the glass walls of the aquarium, where they appear as tiny brown 

 or green cylinders one-half of an inch or more in length. At the free 

 or so-called oral end, a circle of tentacles surrounds a conelike area, 

 the hypostome, in which the mouth is found. The opposite, or aboral, 

 end forms a disklike structure which is provided with mucous cells 

 that aid it in sticking to a surface. Hydras are able to move slowly 

 by a looping motion of the body. The green ones, which are much more 



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