ANNULOIDA ECHINODERMATA. 87 



called the niadreporiform tubercle, because, when examined 

 by a lens, it somewhat resembles a piece of madrepore 

 coral. The ocular plates, of which there are also five, are 

 smaller than the genital, and are placed at the extremities 

 of the ainbulacral passages. Each plate contains an open- 

 ing in which a little eye is placed. 



The plates are furnished with numerous tubercles, to 

 which pines are attached by a ball-and-SOCket joint. 



Fia. 37. PRINCIPAL ORGANS OF SEA-URCHIN. 

 a, oesophagus ; & e f, intestine ; g, anus ; 7i, ovary ; i, shell or test. 



These spines assist in locomotion, and are also used as 

 weapons of defence. Among the spines are found a num- 

 ber of curious appendages called pedicellarijB (Lat. pedi- 

 cellus, a louse), because they were formerly believed to be 

 parasites. They are long and worm-like, and at the head 

 of each are placed three calcareous jaws, which fit together. 

 These jaws continually open and shut. Their object 



