338 EVENINGS AT THE MICEOSCOPE. 



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 in- 

 sertion 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 gi'eater than that in 

 front, is not without significance, since it implies a 

 thicker muscle at that part, which accords with the cir- 

 cumstance that such would be the insertion of the 

 muscle-band whose contraction produces the outnoa/rd 

 stroke by which the sand is forced away from the 

 bed. 



But what is the need of so much care being be- 

 stowed 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 

 movement ? The hairs of our head we cannot move 

 individually : 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 goiag 

 carefully, with the lens at your eye, over the shell, you 

 perceive that the spines, though all formed on a com- 

 mon model, differ considerably in the detail of their 

 form. I have shown you what may be considered the 

 average shape ; but in some, especially the finer ones 

 that clothe the sides, the club is slender and pointed ; 

 in others, as in those behind the mouth, which are the 

 largest and coarsest of all, the club is dilated into a long 



