110 THE HALL OF SHELLS. 
porcupine it is called from the forest of spines 
which cover its test. When dead these spines 
rub off and the beautiful shell is apparent, 
which then is not mappropriately called sea 
egg. Sichinus is the name science has given it. 
This class of radiate animals belongs to the 
sroup Lchinodermata, which means spiny- 
skinned, and truly the Achinide 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. Hach 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 fits over this point m a man- 
ner similar to the ball-and-socket jomts in the 
human shoulder and hip. 
“Tt is by help of these spines that the Achi- 
nus climbs even a smooth surface, or with 
them excavates for itself a hiding place in the 
sand. Many Achini 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 
