IV. ECniNOIDEA. 



513 



Class IV. Echinoidea (Sea Urchins). 



The structure of the sen urchins is best understood in the 

 spherical forms (figs. 328, 330). 

 Moutli and anus lie at opposite 

 poles of the main axis, each open- 

 ing immediately surrounded hy 

 areas covered by calcareous plates, 

 the arrangement of which varies 

 with the family. Around the anus 

 is the periproct, around the mouth 

 the peristome, the latter bearing 

 sphasridia and in the Echinoids five 

 pairs of interambulacral gills. Be- 

 tween peristome and periproct the 

 body wall is composed of calcareous 

 plates, which, except in the Echino- 

 thuridas, are immovably united. 

 Aside from the extinct Pahechei- 

 noidea the plates are arranged in twenty meridional rows, or, more 

 accurately, in ten double rows, two rows being always intimately 

 associated together. Five of these double rows are ambulaci'al. 



Fro. !i-2».—rirliipIciirvxflork!ainis.* (After 

 Agassiz.) Aboral view, the spines 

 removed to sliow the ambulacral (a) 

 and (/)) interambulacral areas, end- 

 ing respectively in the ocular and 

 genital plates ; in the centre the four 

 plates of the periproct. 



Fig. 339. 



Fig. '.m. 



Fig. Z'2f^.~Clypfa^tcr ^iihiji^prcss'its. (After Agassiz.) Aboral view, showing the peta- 



loid ends of the ambulacral areas. 

 Fk;. 3:3(1.— Diagrammatic longitudinal section through a sea urchin. 



the alternating five interambulacral. Both bear small hemisiiheri- 

 cal articular surfaces on which are situated the spines, either long 

 and jiointed or swollen to spherical plates. These spines are ex- 

 tremely mobile and are moved by muscles so that they serve both as 

 protecting and locomotor structures. The ambulacral plates are 



