224 HOMOLOGIES. 
represented by few types, those types are wonder- 
fully rich and varied; but in proportion as other 
expressions of the same structure are introduced, 
the first dwindle, and, if they do not entirely dis- . 
appear, become at least much less prominent than 
before. 
There remain only two other Orders to be con- 
sidered, the Ophiurans and the Holothurians. 
The Ophiurans approach the Crinoids more 
nearly than any other group of Echinoderms, 
and in our classifications are placed next above 
them. In them the ab-oral region, which has 
Ophiuran; showing one ray from the oral side. 
such a remarkable predominance in the Crinoid, 
has become depressed: it no longer extends into 
a stem, nor does it even rise into the calyx-like 
