ITS MECHANISM. 17U 



increase, both of thickness and superficies, the form 

 characteristic of that individual piece is maintained 

 with immutable mathematical precision. Thus the 

 volume and capacity of the box grow with the growth 

 of the individual segments, and it ever keeps the globose 

 shape at first imposed upon it. 



But this is but a small part of the mechanism of this 

 interesting tribe. If you put into a basin of sea-water 

 one of the pretty kind 1 which we find so abundantly 

 under stones at low water, whose green spines are 

 tipped with rosy purple, like the tentacles of an Anthea, 

 you will presently observe it marching majestically 

 along by means of the hundreds of sucker-feet, which 

 it possesses in common with the Star-fish. Now, if you 

 have in your cabinet the empty box of an Urchin of 

 this same kind, and taking it in your hand, hold it up 

 to the light, and look into the cavity from the under 

 or mouth side, you will have a very interesting spec- 

 tacle. The light streams in through a multitude of 

 minute holes, as smooth and regular as if drilled with 

 a fairy's wimble ; and these holes are arranged in a 

 pattern of elegant symmetry. They run in lines, like 

 meridians, from pole to pole of the flattened globe ; but 

 instead of being set at uniform intervals, they constitute 



1 Echinus miliaris, a specimen of which may be seen delineated in 

 Plate xx., in the upper left-hand corner. 



