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, nora 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 Echinide 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 Echinidz. 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 Zchznus 
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 
