282 THE OCEAN WOKLD. 



from a central point on the summit terminating at a point of the sphe- 

 roid diametrically opposite namely, the circumference of the buccal 

 orifice. These vertical zones are of two kinds, some larger and others 

 straighter, each zone consisting of a douhle row of plates, the first 

 charged with movable spines, the second pierced with holes disposed in 

 regular longitudinal series, from which emerge certain fleshy tentacula, 

 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 ; but 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 which 

 the carapace of the urchin presents when the spines are erect and 

 lowered, by reference to Fig. 112 (Echinus mamillatus), which repre- 

 sents the animal bristling with spines, and Fig. 113, in which the same 



Fig. 113. Echinus mamillatus. Sea Urchin, without spines, natural size. 



species 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, to 

 which no other can be compared. To abbreviate slightly Grosse'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. 



