236 



ECHINODERiMS. 



tubercles, and each plate is perforated with the holes 

 for the suckers. The plates which bear the holes are 

 called the a //i/)idirc>-a/ plates, — from a Latin word which 

 means a walk, or alley ; and the large plates without 

 holes are called the intcrainbulacral plates. At the 

 termination of each of the five belts or zones of ambu- 

 lacral plates there is a little triangular plate with a 

 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 inadrc- 

 poric bod}'. It serves as a filter or strainer to the water 

 which passes through it into the water tubes of the 

 animal and which is used in extending the sucker feet. 

 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 Aris- 

 totle's Lantern. In a Sea Urchin of ordinary size there 



are five or six hundred 

 plates, all fitting together 

 in the most perfect man- 

 ner, and bearing more 

 than four thousand 

 spines ; and the suckers 

 number nearly two thou- 

 sand. 



Besides the spines and 

 the suckers, there are 

 scattered over the body 

 and around the mouth 

 of Sea Urchins a great 



Fig. 465- — Echinar.ichnius. Norih- 

 east coast of North America. 



