278 THE OCEAN WORLD. 



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 unchanging 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 the polyhedron, grow 

 together, and the form characteristic of the individual is maintained 

 with immutable mathematical precision. 



The dimensions and shape of the spines are very variable. In 

 certain Echinidae they are three or four times the diameter of the 

 body. In the common sea-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 spines, notwithstanding its 

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



In the spines and ambulacral feet we see the external organs of 

 the Echinidae. The former are instruments of defence ; the latter, 

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

 considered that each of these spines is put in motion by several 

 .muscles, it is impossible to repress our wonder and surprise at the 

 prodigious number of organs brought into action in the sea-urchin. 

 More than 1,200 spines have been counted upon the shell of Echinus 

 esculentus, a representation of which is given in Fig. 113. If we add 

 to this first supply of spines other smaller and in some sort accessary 

 spines, we shall arrive at a total of 3,000 prickles. Each sea-urchin 

 thus bears as many weapons as ten squadrons of lancers. When it 



