METAZOA. 



All remaining divisions or groups of animals are united 

 under the name Metazoa for the following reasons: A 

 careful consideration of their structure leads to the con- 

 clusion 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 neces- 

 sary 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 degener- 

 ate parasites) there is an opening 

 (mouth) for taking in of food and an 

 alimentary tract for its digestion; in 

 all there is a more or less distinct ner- 

 vous system; and in all parts of the 

 body are more or less specialized for 

 respiration. 



A little deeper insight leads to 

 another conclusion which further justi- 

 fies the group of Metazoa. The body 

 is composed of layers, at least two in 

 number, one on the outside forming the 

 skin, and a second on the inside forming 

 the lining of the digestive tract. To 

 these two layers are fiven names, ectoderm and entodcrm 



(fig. 9), meaning respectively outer and inner skin. 



154 



ec 



FIG. 9. Diagram of a 

 two-layered animal, 

 based upon a hy- 

 droid. ec, ectoderm ; 

 en, entoderm. 



