334 EVENINGS AT THE MICROSCOPE 



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

 ually: 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 at your eye, over the shell, you perceive that the 

 spines, though all formed on a common model, differ con- 

 siderably in the detail of their form. I have shown you 

 what may be considered the average shape; but x 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 flat spoon: while in the long, much- 

 bowed spines which densely crowd upon the back, the 

 form is almost uniformly taper throughout and pointed. 

 The animal sinks into the sand mouth downward. The 

 broad spoons behind the mouth come first into requisi- 

 tion, and scoop away the sand, each acting individually 

 and throwing it outward. Observe how beautifully they 

 are arranged for this purpose! diverging from the median 

 line with the curve backward and outward. Similar is 

 the arrangement of the slender side-spines; their curve is 

 still more backward, the tips arching uniformly outward. 

 They take, indeed, exactly the curve which the fore-paws 

 of a mole possess only in a retrograde direction, since 

 the Urchin sinks backward which has been shown to be 

 so effective for the excavating of the soil and the throw- 

 ing of it outward. Finally, the long spines on the back 

 are suited to reach the sand on each side, when the creat- 

 ure has descended to its depth, and by their motions work 

 it inward again, covering and concealing the industrious 

 and effective miner. 



