318 EVENINGS AT THE MICROSCOPE 



It is by making a thin transverse section of a spine - 

 cutting off a slice of it, to speak in homely phrase that 

 we shall demonstrate the structure, which is very beautiful. 

 This is an operation requiring much delicacy and practice, 

 and implements for the special purpose; and hence it is 

 best performed by professional persons, who prepare micro- 

 scopic objects for sale. You may see such a section, how- 

 ever, on this slide; but I do not know whether the spine 

 belongs to the species we are examining. 



The whole central portion is formed of the sponge-like 

 calcareous matter, which, from the variously reflected and 

 refracted rays of light, appears nearly opaque, and of a 

 bluish color by transmitted light. This structure sends 

 forth radiating points (making longitudinal ridges, of 

 course, in the perfect spine); and it is the opacity of these 

 points (or ridges) which reach the circumference that gives 

 to the spine the appearance of being fluted. Indeed it 

 would be fluted if this were the entire structure; but the 

 open space left between these projecting radii is filled with 

 the solid glassy matter, having, as we see, a convex sur- 

 face. This, however, from its perfect transparency, is not 

 seen when we look at the side of the spine, the eye going 

 down to the bottom of the interspace. 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 mech- 

 anism 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 demonstrate, viz. , the mode of articulation. 



