ECHINIDE. 277 
each zone consisting of a double row of plates, the first charged with 
movable spines, the second pierced with holes disposed in regular 
longitudinal series, from which emerge the ambulacral feet, which, as. 
‘we shall see presently, serve as organs of locomotion to the animal. 
When armed with these bristling spines, the sea-urchins resemble the 
hedge-hogs; but when the spines are rubbed off, they look very much 
like a melon or an egg, to which their shape and calcareous nature 
have sometimes led to their being compared by the vulgar as well as 
by the learned. We shall give a tolerably exact idea of the two 
different aspects which the corona of the urchin presents when the 
spines are still on and when they have been removed, by reference to 
Fig. 112.—Acrocladia mamillata. Sea Urchin, without spines, natural size. 
Fig. 111 (Acrocladia mamillata), which represents the animal bristling 
with spines, and Fig. 112, in which the same species is represented 
after death, when these weapons of defence have been rubbed off— 
and how complicated these organs of defence must be! It has been 
calculated that more than ten thousand pieces, each admirably arranged 
and united, enter into the composition of the shell of the sea-urchin. 
To abbreviate slightly Gosse’s description of that wonderful piece of 
mechanism, the sea-urchin : “A globular hollow box has to be made, 
of some three inches in diameter, the walls of which shall be scarcely 
thicker than a wafer, formed of unyielding limestone, yet fitted to hold 
the soft tender parts of an animal which quite fills the cavity at all ages.. 
But in infancy the animal is not so big as a pea, and it has to attain. 
its adult dimensions. The box is never to be cast off or renewed ; the 
same box must hold the infant and the veteran urchin. The limestone 
can only increase in size by being deposited. Now the vascular tissues. 
