228 HOMOLOGIES. 



them, we may fail to recognize the identity of 

 parts, or, at least, it will not strike ns 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. 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 ol 

 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 tc 

 these animals, and to have no affinity with any 

 corresponding feature in the same Class. But a 

 closer investigation has shown them to be onlj 



