18 



CLASSIFICATION AND CREATION. 



The internal cavity and the radiating tubes, in- 

 id of being connected with the outer wall of 



Common Sea-Urchin, Echinus, seen from above. 



the body as in Polyps, or hollowed out of the 

 substance of the body as in Jelly-Fishes, are here 

 enclosed withiu independent walls of their own. 



Echinarachnius, opened by a transverse r- n0 rizontal section, and showing 

 the internal arrangement : o, mr u[n . e e e e e, ambulacra, with their rami- 

 fications cm cm cm ; w w \. w interambulacra. 



quite distinct f rom the wall of the body. But 

 notwithstanding this difference, a transverse sec- 



