ECHINODERMATA. 283 



The box is never to be cast off or renewed ; the same box must hold 

 the infant and veteran urchin. The limestone can only increase in 

 size by being deposited. Now the vascular tissues are within, and the 

 particles they deposit must be on the interior walls. To thicken the 

 walls from within leaves less room in the cavity ; but what is wanted is 

 more room, ever more and more. The growing animal feels its tissues 

 swelling day by day, by the assimilation of food. Its cry is, ' Give 

 me space ! a larger house, or I die !' How is this problem solved ? 

 Ah ! there is no difficulty. The inexhaustible wisdom of the Creator 

 has a beautiful contrivance for the emergency. The box is not made 

 in one piece, nor in ten, nor a hundred. Six hundred distinct pieces 

 go to make up the hollow case ; all accurately fitted together, so that 

 the perfect symmetry of the outline remains unbroken ; and yet, thin 

 as their substance is, they retain their relative positions with un- 

 changing exactness, and the slight brittle box retains all requisite 

 strength and firmness, for each of these pieces is enveloped by a layer 

 of living flesh ; a vascular tissue passes up between the joints, where one 

 meets another, and spreads itself over the whole exterior surface." 



This being so, the glands of the investing tissue secrete lime from 

 the sea water, and deposit it after a determinate and orderly pattern 

 on every part of the surface. Thus the inner face, the outer face, and 

 each side and angle of polyhedron, grow together, and the form charac- 

 teristic of the individual is maintained with immutable mathematical 

 precision. The dimensions and shape of these prickles are very vari- 

 able. In certain Echinidae they are three or four times the diameter 

 of the body. In the urchin, properly so called, they are only three- 

 fourths or four-fifths that diameter. They sometimes resemble short 

 bristles. These defensive weapons have tubercles for supports, which 

 are arranged on the surface of the animal with perfect regularity. At 

 the base they present a small head separated by compression. This 

 head is hollow on its lower face, presenting a cavity adapted to a 

 tubercle of the shell. Each of the prickles, notwithstanding its 

 extreme minuteness, is put in action by a muscular apparatus. 



In the prickles, or spines and tentacula (ambulacra, feet suckers), 

 we see the external organs of the Echinodermata. The former 

 are instruments of defence and progression ; the latter, strange 

 as it may appear, serve them to walk with. When it is considered 

 that each of these prickles is put in motion by several muscles, it is 



