332 EVENINGS AT THE MICROSCOPE 



of the microscope, using reflected light and a dark back- 

 ground. They thus present a very beautiful appearance; 

 elegantly- formed curved clubs, made of a substance which 

 seems to be between glass and ivory, having the whiten.ess 

 of the latter and the glittering brilliance of the former. 

 The entire surface appears to be exquisitely carved with 

 excessively minute oval pits, arranged in close- set lines, 

 with the most charming regularity. It is the light re- 

 flected from the polished bottoms of these pits that im- 

 parts to the surface its sparkling brilliancy. At the bot- 

 tom of the spine there is a little depression, which fits a 

 tiny nipple on a wart-like prominence of the shell, as we 

 saw in Echinus; but a little way above this point there is 

 a singular projection or shoulder of the calcareous sub- 

 stance, which is set on at a very oblique angle with the 

 axis of the spine, reminding one, as we look at the spine 

 laterally, of the budding tines on the horn of a young 

 deer. 



At first, perhaps, you are at a loss to know what pur- 

 pose this shoulder can serve; but by turning to the shell 

 and carefully observing the spines in their natural connec- 

 tion with it, you will observe that the obliquity of its 

 position accurately corresponds with the angle which the 

 individual spines form with the surface of the shell from 

 which they spring; and that the shoulder has its plane 

 exactly parallel with the latter, but raised a little way 

 above it. Now the entire shell, during life, was clothed 

 with a living vascular flesh, having a thickness exactly 

 corresponding to the distance of the shoulder from the 

 shell. This shoulder, then, was an attachment for the 

 muscular bands, whose office it was to move the spine to 

 and fro; the projection affording the muscles a much bet- 



