HOMOLOGIES. 217 
Urchin, only that in the Star-Fish these regions 
are coequal in extent, while in the Sea-Urchin 
the ab-oral region is very contracted, and the 
oral region, with the parts belonging to it, occu- 
pies the greater part of its surface. 
Such being the identity of parts between a 
Star-Fish and a Sea-Urchin, let us see now how 
the Star-Fish may be transformed into the Pe- 
dunculated Crinoid, the earliest representative of 
its Class, or into a Comatula, one of the free ani- 
mals that represent the Crinoids in our day. 
We have seen that in the Sea-Urchins the ab- 
oral region is very contracted, the oral region 
and the parts radiating from it and forming the 
sides being the predominant features in the 
structure; and we shall find, as we proceed in 
our comparison, that the different proportions of 
these three parts, the oral and ab-oral regions and 
the sides, determine the different outlines of the 
various Orders in this Class. In the Sea-Urchin 
the oral region and the sides are predominant, 
while the ab-oral region is very small. In the 
Star-Fish, the oral and ab-oral regions are 
brought into equal relations, neither preponder- 
ating over the other, and the sides are compressed, 
so that, seen in profile, the outline of the Star- 
Fish is that of a slightly convex disk, instead 
of a sphere, as in the Sea-Urchin. But when we 
come to the Crinoids, we find that the great pre- 
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