RADIATES. 309 
588. The Sea Egg, as it is commonly called, is the crust 
or shell of a spiny or prickle-skinned animal, stripped of 
its spines. In Fig. 267 you see this animal, called an 
Fig. 267.—Shell of Echinus, or Sea Urchin; on the right side covered with spines, 
on the left the spines removed. 
Echinus, with the spines removed from half of it. These 
spines are curiously jointed with the shell. There is a 
round projection of the shell at the root of each spine, 
upon which the spine works with its cup-like cavity, mak- 
ing a regular ball and socket joint. These projections 
every one must have noticed arranged with such beauti- 
ful regularity on the Sea Egg. There are the same tubu- 
lar feet as in the Starfish, but much larger, and therefore 
more efficient in taking prey. In walking, while the 
suckers are the moving power, the animal is carried for- 
ward on the spines, these acting after the manner of a 
crutch. The animal inside of this singular shell has a 
stomach, a respiratory apparatus, intestines, etc. Its 
mouth has quite formidable teeth. Small Crustacea and 
Mollusca are its chief food. 
589. The shell is made up of small plates, and, as the 
animal grows, each one of these plates is made larger by 
increase at its edge. The growth is like that of the cov- 
