22 THE ORIGIN OF MAN 



understand more thoroughly the evolution (of 

 animal life. 



Animals with radial symmetry are classified 

 into two divisions, the first being the more simply 

 formed or primitive one and which is the stock 

 (phylum) Cnidaria, or Coelenterata, the latter 

 implying that the animals have but one internal 

 cavity which serves both as a body cavity 

 (coelome) and as a digestive cavity or sac (en- 

 teron). All higher animals have two cavities, a 

 true body, one which may contain the internal 

 organs, such as the heart and lungs, and within 

 this another independent digestive cavity, such 

 as the stomach or intestine. These higher ani- 

 mals are known as coelomates. The coelenter- 

 ates are very simple animals in which appear the 

 beginnings of definite organs and as many of 

 them are flower-like they have been called zoo- 

 phites or plant animals. 



The second division is the phylum Echinoder- 

 mata which includes animals that have marked 



