25 



PHYLUM ECHINODERMA. 



by five "ocular " and five "genital" plates. In the adult 

 the central plate is no longer distinct. The ocular or radial 

 plates bear eye specks ; the genital or basal plates bear the 

 apertures of the genital ducts, and one of the five bears the 

 perforated madreporic plate. From pole to pole run ten 

 meridians of calcareous plates, which fit one another firmly ; 

 five of these (in a line with the ocular plates) are known as 



FIG. 119. Apical disc of sea-urchin. 



In the centre is the periproct bearing the anus; around it are five 

 genital or inter-radial plates (f.)> one of which is modified as the 

 madreporite {/.) ; beyond these five ocular or radial plates (<?.) ; 

 i.r.a, an inter-radial or inter-ambulacral area, with spines 

 only; r.a., a radial or ambulacral area, with spines and open- 

 ings for tube-feet. 



ambulacral areas, for through their plates the locomotor 

 tube-feet are extruded ; the five others (in a line with the 

 genital plates) are called inter-ambulacral areas, and bear 

 spines, not tube-feet. Altogether, therefore, there are ten 

 meridians, and each meridian area has a double row of 

 plates. On the dry shell from which the spines have been 

 scraped, the ambulacral plates are seen to be perforated by 

 small pores, three pairs or so to each plate. Through each 



