RADIATES. 339 



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. 26T. 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 efiicient 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- 



