THE SHAPES AND SIZES OF ANIMALS 139 



quently any light that can be obtained from a study 

 of embryology would be peculiarly welcome. 



It is impossible to discuss the question at all 

 adequately here, but it is perhaps worth while 

 pointing out that though the adult starfish is an 

 animal showing radiate symmetry in a marked 

 manner, yet that the radiate symmetry of a starfish, 

 or indeed of any other Echinoderm, differs in some 

 important respects from the radiate symmetry of 

 the Coelenterate. Thus, in the first place, the 

 symmetry of an Echinoderm is pentamerous, the 

 typical number of arms in a starfish being five, and 

 five being the typical number of corresponding 

 parts met with in the other groups of Echinoderms 

 also. This may seem an unimportant point, but it 

 becomes significant when we note that though the 

 actual number of corresponding parts in a Coelen- 

 terate varies very greatly in different forms, yet 

 that it is almost invariably either four or six, or 

 some multiple of these numbers, while five or any 

 multiple of five is unknown. Then again, in the 

 actual development of an Echinoderm the radial 

 symmetry of the adult is first shown by a set of 

 organs, the ambulacral system, which is absolutely 

 and entirely unrepresented in Coelenterates ; and it 

 is apparently on this radially arranged ambulacral 

 system that the radial symmetry of other parts and 

 systems is based. When further we bear in mind 

 that the whole structure of an Echinoderm is alto- 

 gether different to that of a Coelenterate, and in 

 many respects very much more complex, any real 

 comparison between the two groups becomes very 



