STRUCTURE OF SEA-URCHIN 3OI 



the central plate is no longer distinct. Each of the 

 " oculars " has a hole for a sensitive tube-foot ; the genitals 

 bear the apertures of the genital ducts, and one also 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 ambulacral areas, for through their plates the 

 locomotor tube-feet are extruded ; the five others (in a Ime 

 with the genital plates) are called inter-ambulacral areas, 



Fig. 163. — Apical disc of sea-urchin. 



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

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

 madreporite (m.) ; between these five ocular or radial plates (o.) ; 

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

 r.a., a radial or ambulacral area, with .spines and openings for 

 tube-feet. 



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 pair of pores a tube-foot is connected with 

 an internal ampulla. In the starfish the ambulacral areas 



