CLASSIFICATION AND CEEATION. 49 



tion shows in these animals, as distinctly as in all 

 the rest, the radiating structure typical of the 

 whole branch. In these three classes we have 

 no difference of plan, nor even any modification 

 of the same plan, for either one of them ex- 

 presses it as clearly as any other, but simply 

 three different ways of executing one and the 

 same structural idea. 



To' those already familiar with these animals 

 some technical details showing the absolute iden- 

 tity of structural plan in these three classes of 

 Radiates may not be uninteresting. 



Let us therefore return to the Polyps, and look 

 at the Sea-Anemone in a new aspect. Sup- 

 pose the inner sac to be turned out; it will 

 then present the appearance of a bottle-shaped 

 body, with a row of hollow projections around 

 the base of the neck ; the neck itself being the 

 stomach turned outward, so that its inner surface 

 becomes its outer surface. If we now compare 

 this with the Jelly-Fish, placing both in the same 

 attitude, with the mouth either downward or 

 upward, it becomes apparent that the^ so-called 

 arms surrounding the mouth of the Jelly-Fish 

 correspond exactly to the neck of our bottle- 

 shaped animal, with the sole difference that it 

 is split into lobes in the Jelly-Fish, instead of 

 remaining tubular as in the Polyp. There are, 

 however, many Jelly-Fishes in which it is strictly 



