ECHINODERMATA. 
285 
box is never to be cast off or renewed ; the same box must hold 
infant and veteran urchin. The limestone can only increase in 
s ' z e by being deposited. Now the vascular tissues are within, and the 
Particles they deposit must be on the interior walls. To thicken the 
' Va Us from within, leaves less room in the cavity ; but what is wanted is 
room, ever more and more. The growing animal feels its tissues 
Celling day by day, by the assimilation of food : its cry is, 1 Give 
llle space ! a larger house, or I die !’ How is this problem solved ? 
' ! there is no difficulty. The inexhaustible wisdom of the Creator 
a beautiful contrivance for the emergency. The box is not made 
1,1 one piece, nor in ten, nor a hundred. Six hundred distinct pieces 
,e >° to make up the hollow case ; all accurately fitted together, so that 
perfect symmetry of the outline remains unbroken ; and yet, thin 
as their substance is, they retain their relative positions with nn- 
c Vgi ng exactness, and the slight brittle box retains all requisite 
length and firmness, for each of these pieces is enveloped by a layer 
of Uvin g flesh ; a vascular tissue passes up between the joints, where one 
^ets another, and spreads itself over the whole exterior surface. 
Phis being so, the glands of the investing tissue secrete lime from 
tlle sea water, and deposits it after a determinate and orderly pattern 
every part of the surface. Thus the inner face, the outer face, and 
| ae h side and angle of polyhedron, grow together, and the form charac- 
teri «ti c of the individual is maintained with immutable mathematical 
P^cision. The dimensions and shape of these prickles are very vari- 
a P®- In certain Echinida: they are three or four times the diameter 
^ the body. In the urchin, properly so called, they are only three- 
'"’fths or four-fifths that diameter. They sometimes resemble short 
Misties, These defensive weapons have tubercles for supports, which 
l!' 1 Pranged on the surface of the animal with perfect regularity. At 
, ‘ e We they present a small head separated by compression. This 
, Ga( P is hollow on its lower face, presenting a cavity adapted to a 
terete of the shell. Each of the prickles, notwithstanding its 
ext rem e minuteness, is put in action by a muscular apparatus. 
P’ 1 the prickles, or spines and tentacula (ambulacra, feet suefers) 
' ,Ve 8 ®e the external organs of the Echinodermata. The former 
rlle instruments of defence and progression ; the latter, strange 
^ ma y appear, serve them to walk with. When it is considered 
lat each of these prickles is put in motion by several muscles, it is 
