CHAPTEE VIII 

 CCELENTERATA 



Many- Celled Animals. — Many-celled animals prob- 

 ably owe their existence to some accident which pre- 

 vented complete fission. The cells after division re- 

 mained attached. Those that were subjected to different 

 environment assumed different functions, and became 

 different in structure. Those which developed a similar 

 structure with power to do the same kind of work became 

 banded together in groups called tissues or organs. 



]\I any-celled animals have in this way developed cer- 

 tain characteristics in common. The cells of which they 

 are composed have been subjected to the same kind of 

 influences and have undergone development in the same 

 general directions. In all many-celled animals, there- 

 fore, the same tissues are present. When these tissues 

 are once established their development becomes one of 

 degree, not of kind, and the quality of the work that 

 they are able to accomplish becomes dependent merely 

 on the degree of development. 



The Primary Layers. — The cells were first arranged 

 in two layers. The outside layer, or ectoderm, formed 

 the skin and was protective in nature ; the inside layer, 

 or entoderm, formed the lining of the digestive tract and 



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