178 JULY. 



together, that the perfect symmetry of the outline is 

 not broken ; and yet, thin as their substance is, they 

 retain their relative position with unchanging exactness, 

 and the slight brittle box possesses all requisite strength 

 and firmness. 



Each of these symmetrical pieces of shell is enveloped 

 by a layer of living flesh, a vascular tissue of exceeding 

 thinness, which passes up between the joints where one 

 meets another, on every side, and not only so, but ac- 

 tually spreads itself over the whole exterior surface. So 

 that when you take up an Urchin into your hand, and 

 having rubbed a small space clear of spines, look on it ; 

 you have not, as you may suppose you have, exposed 

 the surface of the shelly box, but only the flesh that 

 covers it ; yet this is so transparent and colourless, so 

 inconceivably thin, so absolutely adherent at every 

 point, that its presence will not be discernible to feel- 

 ing or sight, without the aid of high microscopic 

 powers. 



This being so, the glands of the investing fleshy 

 tissue secrete lime from the sea- water, which holds it 

 in solution, and constantly deposits it, after a determi- 

 nate and orderly pattern, on every part of the surface of 

 each shelly piece ; the inner face, the outer face, and 

 each of the sides and angles of the polyhedron grow 

 together, and all so evenly, that while the dimensions 



