HOMOLOGIES. 227 



by naturalists ; but as soon as their true 

 structure was understood, agreeing in every re- 

 spect with that of the other Echinoderms, and 

 having no affinity whatever with the articulated 

 structure of the Worms, they found their true 

 place in our classifications. 



The natural attitude of these animals is differ- 

 ent from that of the other Echinoderms. They 

 lie on one side, and move with the oral opening 

 forward ; and this has been one cause of the 

 mistakes as to their true affinity. But when we 

 would compare animals, we should place them, 

 not in the attitude which is natural to them in 

 their native element, but in what .1 would call 

 their normal position, that is, such a position as 

 brings the corresponding parts into the same re- 

 lation in all. For instance, the natural attitude 

 of the Crinoid is with the ab-oral region down- 

 ward, attached to a stem, and the oral region or 

 mouth upward. The Ophiuran turns its oral 

 region, along which all the suckers or ambulacra 

 are arranged, toward the surface along which it 

 moves. The Star-Fish does the same. The Sea- 

 Urchin also has its oral opening downward. But 

 the Holothurian moves on one side, mouth fore- 

 most, as represented in the preceding wood-cut, 

 dragging itself onward, like all the rest, by means 

 of its rows of suckers. If, now, we compare 

 these animals in the various attitudes natural to 



