GENERAL ORGANIZATION OF THE ANIMAL BODY 243 



Coelomocoela exhibit the gastrula stage {cf. Fig. 148 G, p. 307 and 

 Fig. 209, p. 399) in their development may be taken as evidence for 

 the course of evolution here indicated. The gastrula is a two-layered 

 sac comparable with the Cuelenterata in its type of structure 

 {cf. Fig. 121, p. 250), and perhaps reminiscent of an inheritance from 

 an enterozoan ancestry. In other words, it may be that a gas- 

 trula stage occurs in these higher phyla because they have never 

 completely lost this evidence of their primitive ancestry; and it 

 may be that coelenterates are two-layered because they have never 

 evolved beyond what is fundamentally the enterozoan stage. 

 Similar propositions could be laid down for the one-cell stage in all 

 Metazoa (Fig. 110, p. 215), for the gill-sHt stage in higher verte- 

 brates (Fig. 287, p. 531), and for many features of development 

 that are suggestive of ancestry in the various phyla. According 

 to such an interpretation, the frog (Fig. 213, p. 405) arises from 

 male and female gametes, which fonn a zj'gote, and later passes 

 through its gastrula and gill-slit stages because it has never lost 

 these vague evidences of its ancestral history as a vertebrate 

 and as a member of the Metazoa. Classification upon a basis of 

 structural resemblance has, therefore, a deeper meaning than 

 convenience, since it is fundamentally the study of evolutionary 

 relationships (cf. p. 506). 



General Organization of the Animal Body 



Forms of Symmetry. — In this connection the principal struc- 

 tural differentiations found among animals may be explained. 

 Forms like Volvox (Fig. 106, p. 200) have a universal symmetry, 

 since they are symmetrical around the center of a sphere. Any 

 plane that passes through this center will divide the individual 

 into halves that are symmetrical. A few of the protozoa are thus 

 universal in symmetry, but there are no cases among Metazoa. 



The radial symmetry that exists in animals like the fresh-water 

 polyp, Hydra (Fig. 128, p. 266), and in the starfish (Fig. 118) 

 is characteristic of Coelenterata and Echinodermata. Such 

 animals are symmetrical, like an umbrella or a cj'linder, around 

 a line that is the principal axis of the body. There are a number 

 of planes through this axis that will divide the individual into 

 equal halves. As with plants, radial symmetry is intimately 

 related to an attached mode of life. Radially symmetrical 

 animals that are free-living, like jellyfish and starfish, have 



