228 HOMOLOGIES. 
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 andstem. In the 
same way, if we place them in the same attitude, 
we shall see that the long, straight rows of suck- 
ers along the length of the Holothurian, and the 
arching zones of suckers on the spherical body of 
the Sea-Urchin, and the furrows with the suckers 
protruding from them along the arms of the Star- 
Fish and Ophiuran, and the radiating series of 
pores from the oral opening in the Crinoid, are 
one and the same thing in all, only altered some- 
what in their relative proportion and extent. 
Around the oral opening of the Holothurian there 
are appendages capable of the most extraordinary 
changes, which seem at first to be peculiar to 
these animals, and to have no affinity with any 
corresponding feature in the same Class. Buta 
closer investigation has shown them to be only 
