40 



NA TURA L HISTOR Y. 



ECHINODERMS. 



Isr the older zoological system, that founded by the great French natu- 

 ralist Cuvier, one of the four great groups into which all animals were 

 divided was called Radiates, in allusion to the marked radiate structure 

 which the individuals possessed ; but this was a character of secondary im- 

 portance when compared with many others, and deeper study has conclu- 



Fig. 41. — Common New England Sea-urchin (Strongyloeentrotus) , with 



its tube feet extended. 



sively shown that the group of Radiates was not a natural one, but that it 

 included two very different kinds of animals, — one the Ccelenterates just 

 passed ; the other the Echinoderms now before us. 



If we examine one of the typical Echinoderms, like a sea-urchin, the 

 name of the group will be seen to be peculiarly appropriate, for the word 

 Echinoderm means spiny skin, and the urchin is seen to be well provided 



