294 EVENINGS AT THE MICROSCOPE. 



The tubercles on the shell show a structure which cor- 

 responds with this. They are very minute ; but each of 

 them is regularly formed, and is crowned with its little 

 polished nipple, on which, as I have said, the spine works, 

 as by a ball-and-socket joint. These are arranged with 

 perfect regularity in what is called quincunx * fashion ; and 

 by close examination you will see that each is inclosed in a 

 little area formed by a very low and narrow ridge of the 

 shell, which makes a network. On the lateral portions of 

 the under surface the meshes of this net are particularly 

 conspicuous, and we see that they constitute shallow hexa- 

 gonal cells, in the midst of which is seated the tubercle ; 

 yet not in the exact centre either, but nearer the front 

 than the back of the area inclosed. 



Now this elevated ridge affords, doubtless, the insertion 

 of the other end of the muscles that move the spine ; the 

 ridge giving a better purchase than a flat surface, as the 

 keel on the breastbone of birds is deep in proportion to 

 the vigour of the muscles used for flight. And, surely, 

 the apparently trivial fact that the space behind the 

 tubercle is greater than that in front, is not without signi- 

 ficance, since it implies a thicker muscle at that part, 

 which accords with the circumstance that such would be 

 the insertion of the muscle-band whose contraction pro- 

 duces the outward stroke by which the sand is forced away 

 from the bed. 



But what is the need of so much care being bestowed 

 upon the separate motion of these thousands of hair-like 

 spines, that each individual one should have a special 

 structure with special muscles, for its individual move- 

 ment ? The hairs of our head we cannot move individu- 

 ally : why should the Heart-Urchin move his ? Truly, 

 these hairs are the feet with which he moves. The animal 

 inhabits the sand at the bottom of the sea in our shallow 

 bays, and burrows in it. By going carefully, with the lens 

 * See page 60 for an explanation of this term. 



