HOMOLOGIES. 217 



Urchin, onlj 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 compresses, 

 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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