SEA URCHINS. 



237 



Fig. 466. — Mellita. Southeast coast of United States. 



number of curious little 

 organs called pcdicil- 

 larice. They look like a 

 stem ending in a knob, 

 but the knob is com- 

 posed of three pieces or 

 blades, which open and 

 shut tightly, thus form- 

 ing a sort of pincers. 

 These organs are of use 

 in keeping the shell 

 clean. 



The number of kinds 



Rotula. Coast of Africa. 



Fig. 467. 



of Sea Urchins is quite large, and they vary in size 

 from an inch to three or four inches in diameter, and 

 have spines from a quarter of an inch long to three or 

 four inches in length. Some of them are capable of 

 making holes in hard substances, even in limestone and 

 granite. 



