"^^^ THE NEW EVOLUTION T"^ 



In one group of animals with the body composed of three cell 

 layers, the ctenophores (fig. 66, p. iii), the symmetry is quadri- 

 lateral; there are two axes, a long and a short, crossing each other 

 in the middle at right angles, and the two halves on opposite sides 

 of either axis are the same. These curious creatures (sec fig. B, 

 p. 140, fig. C, p. 146, and fig. D, 4, p. ^50) may be regarded as on 

 a par with the coelenterates, though wholly different from them. 



All other animals are always in some stage, and usually in the 

 adult, bilaterally symmetrical with a more or less marked head 

 end at which are the main nerve centers, the chief sense organs, 

 and usually the mouth, if a mouth be present. 



The bilaterally symmetrical animals segregate themselves in 

 two rather well defined groups. The first group includes those 

 types in which the symmetry is partly bilateral and partly radial, 

 and there is no typical gastrula or cup stage in the development of 

 the individuals. The second group includes the types in which 

 the symmetry is wholly bilateral, and which in their development 

 pass through a typical gastrula stage, or a stage easily recognized 

 or interpreted as a modification of the gastrula. 



The animals which are partly bilateral and partly radial in 

 their symmetry form a most extraordinary assemblage of very 

 widely diverse types. They are the cestodes or tapeworms, the 

 acanthocephalids or spiny-headed worms, the trematodes or 

 flukes, the turbellarians, the nematodes or thread-worms, the 

 nematorpha or gordian worms (the "hair-snakes" of puddles and 

 troughs), and a considerable number of curious small obscure 

 worm-like things of uncertain relationships. Although the only 

 features which these queer creatures share in common are the 

 partially bilateral and partially radial symmetry, the absence of 

 true ccelomic structures, and the absence of a true gastrula stage 

 in the development of the individuals, there are indications that 

 as a whole they represent a distinct zoological unit, although 

 a remarkably heterogeneous unit. 



There are forms which are intermediate between the tape- 

 worms and the flukes, and others which are intermediate between 

 the flukes and the turbellarians. Some authorities consider the 

 spiny-headed worms to be related to the nematodes or thread- 

 worms, others believe them to be most nearly allied to the tape- 

 worms, while still others regard them as wholly distinct from 

 anything else. The gordian worms are usually considered to be 



[2.42-] 



