136 THE SHAPES AND SIZES OF ANIMALS 



with many zoologists, but which can at present 

 hardly be regarded as more than an hypothesis. 



With regard to the mechanical origin of bilateral 

 symmetry suggested above, Herbert Spencer, 

 whose writings on the fundamental laws govern- 

 ing animal form and structure are of the greatest 

 possible interest, speaks as follows : " Where the 

 movements subject the body to different forces at 

 its two ends, different forces on its under and upper 

 surfaces, and like forces along its two sides, there 

 arises a corresponding form, unlike at its ex- 

 tremities, unlike above and below, but having its 

 two sides alike." 



We have seen above that there are reasons of 

 very great weight for regarding the radiate type as 

 more primitive than the bilaterally symmetrical one, 

 and further than this, for regarding the latter as 

 directly descended from the former. The group of 

 Echinodermata, including the starfish, brittle stars, 

 sea urchins, and their allies, warn us however that 

 we must not generalise too hastily. About the 

 radiate symmetry of an Echinoderm there can be no 

 possible doubt ; a starfish is as markedly, as con- 

 spicuously, radiate as any animal in existence, 

 indeed it has been by older writers taken as the 

 type of radiate animals. Take one of the ordinary 

 five-fingered starfish for instance : note its shape ; 

 the central disc-like body produced into five equal 

 and symmetrically arranged arms, the mouth 

 placed centrally on the lower, or oral surface, the 

 anus subcentrally on the dorsal surface. As re- 

 gards internal structure, the radiate symmetry is 



