SEA-URCHINS. 



they are of important use. Their appetite and instinct lead them to devour 

 whatever dead or tainted substance they may happen to encounter, and thus 

 by their multitudes they prevent our shores from being encumbered by the 

 offal that would otherwise accumulate on every beach. They are, moreover, 

 highly prolific, pouring forth at certain seasons their innumerable eggs into 

 the surrounding water, and thus materially contributing to the supply of food 

 provided for the multitudinous inhabitants of the sea. 



In the Cake-Urchins (Scutelld}* (Fig. 44, 2), the rays are obliterated, and 

 the circumference acquires a circular form; moreover, their body is encased 

 completely in a hard calcareous shell, composed of numerous angular pieces, 

 accurately fitted together and incapable of movement. The cake-urchins 

 bury themselves in the sand, a situation in which suckers would be of little 

 use, but for which they are admirably adapted by a contrivance not less cal- 

 culated to excite the admiration of the observer. 



The exterior of the Scutella is entirely covered with minute appendages, 

 resembling, when seen with the naked eye, delicate hairs; but, when examined 

 with the microscope, they are found to be spines of most elaborate structure, 

 all of which are moveably attached to the shell by ball-and-socket joints, and 

 thus rendered susceptible of being moved in every direction, so that by their 

 combined efforts the animal can speedily bury itself, either for the purpose of 

 procuring food or of eluding observation. 



The Sea-Eggs or Sea-Urchins (Echinus) f (Fig. 44, i) in their form 

 resemble an orange. The mouth is a simple orifice in the shell, placed at one 



FIG. 47. GREEN-PEA URCHIN. 



FIG. 48. SUCKER OF URCHIN. 



extremity of its axis, and through it, as represented in the figure, the points of 

 five singular teeth project externally. The instruments of locomotion occupy 

 the entire superficies of the shell, and present two distinct sets of organs 

 adapted to different uses. The first consists of a multitude of sharp purple 

 spines (Fig. 47), every one of which is connected by a ball-and-socket jint to 

 a distinct prominence on which it moves. These numerous spines, therefore, 

 are so many inflexible legs, upon which the Echinus rolls itself from place to 

 place, or by their assistance it can bury itself in the sand with the greatest 

 facility. But these wonderfully constructed animals are by no means confined 



Scutellum, a little shield, 



echmos, a hedgehog. 



