

THE NEW EVOLUTION 





COELEITTSRATE 



Body radially symmetrical, 

 with definite organs. 









Body a cellular mass, 

 with no definite organs. 



Fig. a. — Diagram illustrating the three simultaneous paths 

 of development from the primitive single cell. Although both 

 the protozoans and the sponges are enormous groups including 

 great numbers of species, neither group ever gave rise to any 

 other type. The geometrical line of development resulting in the 

 radially symmetrical coelenterate leads through the gastrula (see 

 figs. 110-117, p. 185), an embryonic stage found in all the more 

 complex animals. The coelenterates and all other animals ex- 

 cept the sponges and the protozoans must therefore be interpreted 

 in terms of the gastrula (see figs. 115, 116, 12.0, p. 185). 



[^38] 



