284 
THE OCEAN WOULD. 
from a central point on the summit; terminating at a point of the spbe* 
roid, diametrically opposite — namely, the circumference of the buccal 
orifice. These vertical zones are of two kinds, some larger and others 
straigliter, each zone consisting of a double row of plates, the firs* 
charged with movable spines, the second pierced with holes disposed m 
regular longitudinal series, from which emerge certain fleshy tentacul® 
which, as we shall see presently, serve as feet to the animal. "When 
armed with these bristling spines, the sea-urchins resemble the hedge' 
hogs ; hut when the spines are down, 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 Avhicb 
the carapace of the urchin presents, when the spines are erect and 
lowered, by reference to Fig. 112 (Echinus mamillafm), which represent 8 
the animal bristling with spines, and Fig. 113. in which the same specie® 
Fig. 113 . Echinus mamillatus. Sea Lichen without spines, natural size. 
is represented after death, when deprived of these weapons of defence : 
and how complicated these defences 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, t° 
which no other can be compared. To abbreviate slightly Gosse® 
description of that wonderful piece of mechanism, the sea-urchin : “ ^ 
globular hollow box has to he made, of some three inches in diameteL 
the walls of which shall be scarcely thicker than a wafer, formed 0 
unyielding limestone, yet fitted to hold the soft tender parts of 
animal which quite fills the cavity at all ages. But in infancy 
animal is not so big as a pea, and it has to attain its adult dimension 8 - 
