140 THE SHAPES AND SIZES OF ANIMALS 



nearly impossible, and we find it less anomalous than 

 it at first appeared to regard the adult symmetry 

 of an Echinoderm as something quite distinct from, 

 and acquired perfectly independently of, that of a 

 Coelenterate. 



Limits both of space and time forbid that I 

 should pursue further the discussion of the shapes 

 of animals. Where once bilateral symmetry is 

 established however the further modifications seen 

 in the higher forms become comparatively easy to 

 follow. The development of a head, with accom- 

 panying concentration of the nervous system and 

 sense organs, are merely further developments of 

 processes and tendencies which we have seen 

 already established in the Turbellarian worms, 

 while the formation of limbs, perhaps shadowed 

 forth in the parapodia of Chaetopods, is the most 

 important step in the upward progress to the 

 highest groups of animals. 



Bilateral symmetry is characteristic of all the 

 higher groups, though it may be masked or modi- 

 fied by further development, as the twisting of the 

 body of a snail or other gastropod, the asymmetrical 

 form of the tail of a hermit crab, or the shifting of 

 the eye in a sole. Speaking generally we may say 

 that the forms of the higher animals are derived from 

 those of the lower bilaterally symmetrical worms, 

 by exaggerating the differences between one part of 

 the body and another already present in these latter. 

 Thus the differences between the dorsal and ventral 

 surfaces or rather halves of the body, and between 

 the anterior and posterior ends of the body, gradually 



