M ZOOGENESIS Ptf^ 



"t^ 



d"^ body radially symmetrical about the central axis only; 

 if the body is in fours, the crossed axes are of the same 

 length and the quadrants are alike; no vibratile 

 plates; four or more tentacles, lobes, or other proc- 

 esses present; stinging cells always present; always 

 with asexual reproduction, at least in the young 

 stages; no mesoderm 



CcELENTERATA 



b"^ no definite organs or structures; body a solid mass of very 

 varied form pierced by innumerable small holes which lead 

 into a system of canals running together and eventually 

 leading into the exterior by one or several large openings 



PORIFERA 



a^ Body composed of a single cell, or the equivalent of a single 

 cell 



Protozoa 



The relationships between the different animal types are of 

 two sorts, differences and resemblances, and the successful divi- 

 sion of the animal types into phyla is dependent upon the proper 

 appreciation of the relative importance of the differences and the 

 resemblances. 



The potentialities of an animal form as an effective mechanism 

 are largely dependent upon the basic symmetry of the form. The 

 basic animal symmetries therefore should be regarded as the 

 dominating factors in animal morphology to which all other 

 factors are subordinated. 



In the animal world there are five distinct types of symmetry 

 (fig. D, p. 150). These five types of symmetry are characteristic 

 of (i) all germ cells and all protozoans at some stage in their life 

 history; (z) the Coelenterata; (3) the Ctenophora; (4) the Vermi- 

 formes; and (5) all other types of animal life. 



The symmetry of the single celled animals or protozoans is 

 based primarily on the symmetry of the sphere — that is, a sym- 

 metry radiating equally in all directions from a central point. 

 But this ideal symmetry is maintained, or closely approached, 

 in only a very few types most conspicuous of which are certain 

 heliozoans and radiolarians. 



The symmetry of the sponges is based upon a mass production 

 of cells which do not become segregated into definite organs. 



[vi] 



