110 THE HALL OF SHELLS. 



porcupine it is called from the forest of spines 

 whicli cover its test. When dead these spines 

 rub off and the beautiful shell is apparent, 

 which then is not inappropriately called sea 

 egg. Echinus is the name science has given it. 

 This class of radiate animals belongs to the 

 group Mchinodermata, vs^hich means spiny- 

 skinned, and truly the EcMnidce are a spiny 

 set. Yet those very spines are most wonder- 

 ful examples of the divine handiwork. Their 

 tints are delicate and various ; the substance 

 of which they are composed is a calcareous 

 matter, but transparent as glass. Each spine 

 is connected with the interior of the animal 

 and moved at its will. You have noticed the 

 tiny raised processes on the surface of a dead 

 urchin's shell; the spines have a depression 

 which exactly tits over this point in a man- 

 ner similar to the ball-and-socket joints in the 

 human shoulder and hip. 



" It is by help of these spines that the Echi- 

 nus climbs even a smooth surface, or with 

 them excavates for itself a hiding place in the 

 sand. Many Echini are able by some means 

 to bore holes in rocks, and there spend their 

 days in seclusion and safety from enemies that 

 infest the sea. 



" The mouth of the sea urchin is on the 



