SEA-URCHINS. 



221 



minute opening which marks the place of the eye. 

 Alternating with these ocular plates, so called, are five 

 larger plates, each being perforated with a larger hole 

 through which the eggs are laid. One of these plates 

 is much larger than the others, and is filled with very 

 minute holes, and is called by naturalists the madre- 

 poric body. It is believed to serve as a filter or 

 strainer to the water which passes through it into the 

 body of the animal. The mouth, at the under side, 

 is armed with five strong pointed and polished teeth, 

 which form the outer part of a remarkable dental 

 apparatus, which is called Aristotle's lantern. In a 

 sea-urchin of ordinary size there are five or six hun- 

 dred plates, all fitting together in the most perfect 

 manner, and bearing more than four thousand spines ; 

 and the suckers number nearly two thousand ! 



Besides the spines and the suckers, there are scat- 

 tered over the body and around the mouths of Sea- 

 Urchins a great number of curious little organs called 

 Pedicillarice. 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. 

 The uses of these or- 

 gans are not well un- 

 derstood.- 



The number of kinds 

 of Sea-Urchins is quite 

 large, and they vary in 



Size from an inch to Fig . 4 6 5._Echinarachnius. Northeast 

 three or four inches . coast of North America. 



