ECHINUS — CORONA 



511 



The number and variety of the pedicellariae, then, is an 

 eloquent testimony to the dangers to which the soft sensitive 

 skins of the Sea-urchin and other Echinodermata are exposed,, 

 and afford confirmatory evidence in support of the view ex- 

 pressed above, that the method adopted to defend the skin was 

 one of the great determining features which led to the division of 

 the Asteroidea into different races. 



The corona consists of five radial or " ambulacral " bands of 

 plates and five interradial, or as they are usually termed, " inter- 



FlG 227.— Dried shell oi Hchinus esculentus, showing the arrangement of the plates of 

 the corona, x 1. 1, The anus ; 2, periproct, with irregular plates ; 3, the 

 madreporite ; 4, one of the other genital plates ; 5, an ocular plate ; 6, an mter- 

 ambulacral plate ; 7, an ambulacral plate ; 8, pores for protrusion of the tube-Jeet ; 

 9, tubercles of the primary spines, i.e. primary tubercles. 



ambulacral" bands of plates— ten in all. Each of the ten 

 consists of two vertical rows of plates throughout most of its 

 extent, and each plate is studded with large bosses, or "primary 

 tubercles " for the primary spines, smaller bosses called " second- 

 ary tubercles" for the secondary spines, and finally, minute 

 elevations called " miliary tubercles " for the pedicellariae. Even 

 in the dried skeleton, however, the ambulacral plates can be 



