CHAPTER TWENTY-EIGHT 



Protozoa 



and 



Metazoa 



Endoderm 



and 



ectoderm 



CCELENTERATA 



1. ANIMALS are divided into the Protozoa and Meta- 

 zoa. The Protozoa, as we have seen, consist of single 

 cells, or of aggregations of similar cells. The Metazoa 

 are the multicellular or many-celled organisms, includ- 

 ing, of course, all the higher forms. If we set aside the 

 sponges as representing a quite distinct line of develop- 

 ment, we may recognize in the typical Metazoa certain 

 characteristics common to the whole series, apparently 

 indicating evolution from a single stem. The ccelen- 

 terate, such as the jellyfish or hydra, is essentially a sac 

 with two layers of cells, of which the inner is called the 

 endoderm and the outer the ectoderm. The terms 

 "hypoblast" and "epiblast" are used by authors in the 

 same sense. Now these layers may be seen in the early 

 stages of the highest animals, and in development they 

 form definite structures. Thus the inner layer gives 

 rise to the various parts connected with the alimentary 

 canal, but from the outer is developed the nervous sys- 

 tem. A middle layer, becoming distinctly defined in the 

 higher groups, produces the skeleton of the vertebrate 

 and other important structures. This middle layer 

 (mesoderm or mesoblast) is not present in the ccelente- 

 rates, though materials derived from the two primary 

 layers form a poorly organized mesoglcea lying between 

 them. 



Thus it appears that the basic structure of the higher 

 animals was laid down in the lower Metazoa ; and what 

 was then developed, millions of years ago, conditions the 

 development of man himself today. 



2. On the other hand, the ccelenterates lack very 

 important structures. They possess a single internal 



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