HOMOLOGIES. 225 



or cup-like projection so characteristic of the Cri- 

 noids, though, when the animal is living, the 

 ab-oral side of the disk is still quite convex. 

 The disk in the Ophiurans is small in comparison 

 to the length of the arms, and perfectly circular. 

 It does not merge gradually into the arms, as in 

 the Star-Fish, but the arms start abruptly from 

 its periphery. In these, as in the Crinoids, the 

 interambulacral plates are absent, and the inter- 

 ambulacral spaces are filled by an encroachment 

 of the ab-oral region upon them. There is an 

 infinite variety and beauty both of form and color 

 in these Sea-Stars. The arms frequently measure 

 many times the diameter of the whole disk, and 

 are so different in size and ornamentation in the 

 different Species, that, at first sight, one might 

 take them for animals entirely distinct from each 

 other. In some the arms are comparatively short, 

 and quite simple ; in others they are very long, 

 and may be either stretched to their full length, 

 or partly contracted, to form a variety of graceful 

 curves. In some they are fringed all along the 

 edges ; in others they are so ramified that every 

 arm seems like a little bush, as it were, and, in- 

 tertwining with each other, they make a thick 

 net-work all around the animal. In the geological 

 succession, these Ophiurans follow the Crinoids, 

 being introduced at about the Carboniferous 

 period, and perhaps earlier. They have had 

 10 o 



