HOMOLOGIES. pila 
sified 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 Werms, 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 I 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 ora 
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 
