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 in all into the same
relation. For instance, the natural attitude of the Crinoid is with
the ab-oral region downward, 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
foremost, as represented in the adjoining 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 them, we may
fail to recognize the identity of parts, or, at least, it will not
strike us at once. But if we place them all--Holothurian, Sea-Urchin,
Star-Fish, Ophiuran, and Crinoid--with the oral or mouth side
downward, for instance, we shall see immediately that the small area at
the opposite end of the Holothurian corresponds to the area on the top
of the Sea-Urchin; that the upper side of the Star-Fish is the same
region enlarged; that, in the Ophiuran, that region makes one side of
the small circular disk; while in the Crinoid it is enlarged and
extended to make the calyx-like projection and stem.
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