SEA-URCHINS AND SEA-CUCUMBERS. 



281 



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open space left between these projecting radii is filled 

 with the solid glassy matter, having, as we see, a convex 

 surface. This, however, from its perfect transparency, is 

 not seen when we look at the side of the spine, the eye 

 going down through the transparent matter to the bottom 

 of the space. The spine is, in fact, a fluted column of 

 spongy glass, with the grooves filled with solid glass. 



We have not yet seen, however, the beautiful mechanism 

 appropriated to the movement of these spines. You can 

 hardly see this to advantage in 

 the living animal, but here is 

 the entire shelly box of a dead 

 Echinus, on which, while for 

 the most part the surface is 

 denuded of spines, a few dozen 

 remain sufficiently attached to 

 show what I wish to demon- 

 strate, viz. , the mode of articu- 

 lation. You observe that th« 

 whole globose shell is covered 

 with tiny knobs, differing in 

 size, and not set in very re- 

 gular, or at least not very ob- 

 vious, order, but showing a 

 tendency to run in lines from 

 ;pole to pole of the globe. Giving attention to one of tlie 

 larger of these knobs, under a lens it is seen to be a hemi- 

 spherical eminence on the shell, the very summit of which 

 is crowned by a tiny nipple of polished whiteness, re- 

 sembling ivory. Xow if we carefully lift one of the still 

 remaining spines from its attachment, which in its present 

 •dried state is so fragile that the slightest touch is sufficient 

 for the purpose, we shall note that its base rests on this 

 tiny nipple ; and, on turning it up, and bringing the mag- 

 nifying power to bear upon its base, we see that this is 

 •excavated with a hollow, whose dimensions exactly corre- 



SPINE OF ECHINUS. 



Segment of Section. 



