METAZOA. 



All of the divisions or groups of animals so far studied 

 are united by naturalists under the name Metazoa for the 

 following reasons: A careful consideration of their struc- 

 ture leads to the conclusion that, in all, the body is of 

 appreciable size, and that, in each and every one, certain 

 portions or organs are specialized for the performance of 

 certain functions necessary in the economy of the individual. 

 Thus we find in all reproductive organs which have solely 

 to do with the perpetuation of the species; in all (except 

 a few degenerate parasites) there is a mouth for taking in of 

 food and an alimentary tract for its digestion ; in all there is 

 a more or less distinct nervous system ; and in all, parts of 

 the body are more or less specialized for respiration. 



A little deeper insight leads to another conclusion which 

 farther justifies the group of Metazoa. In 

 all animals so far studied the body is com- 

 posed of layers, at least two in number; 

 one on the outside forming the skin, and a 

 a second on the inside forming the lining 

 of the digestive tract. To these two layers 

 are given names, ectoderm and entoderm, 

 meaning respectively outer and inner skin. 



In the Ccelenterata all of the functions 

 of the animal are performed by either one 

 or the other of these two layers. In all 

 the other divisions a third layer occurs FIG. 145. Diagram of 



a two-layered ani- 



between ectoderm and entoderm the mai, based upon a 



nydroid. ec, ecto- 



mesoderm (middle skm), and this meso- derm; en, entoderm. 

 derm takes some of the functions which are divided between 



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