SEA-URCHINS AND SEA-CUCUMBERS. *293 



sometimes called the Hairy Sea-egg. The array of spines 

 has a glittering silky appearance in this dried state. 



We will now put a few of them under a low power 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 whiteness 

 of the latter and the glittering brilliance of the former. 

 The entire surface appears to be exquisitely carved, with 

 very minute oval pits, arranged in close-set lines, with the 

 most charming regularity. It is the light reflected from 

 the polished bottoms of these pits that imparts to the sur- 

 face its sparkling brilliancy. At the bottom 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 pro- 

 jection or shoulder of the calcareous substance, 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 con- 

 nexion 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 flesh, having a thickness exactly correspond- 

 ing 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 pro- 

 jection affording the muscles a much better purchase, or 

 power, than they could have had if they had been inserted 

 into the slender stem itself. 



