IX 



PHYLUM ECHINODERMATA 



305 



the case of one of these sets of zones the ambulacra! zones or 

 ambulacral areas already referred to (amft) each of the plates is 

 perforated towards its outer end by two minute pores, the ambulacral 





-J,..'- -::-J- - >..v 



i 



M3 5 j J/- '4 *tMsB J ^?? /= ' ' 



Int.asttb 



Am.b 



amb 



FIG. 204. Corona of Sea-ui chin with the spines removed to show the arrangement of the^ 

 plates, lateral view. An.li. ambulacral zone with its perforated plates; Ap. apical (abural) 

 pole ; Int. antb. inter-anil ulacral zones. (From Bronn's Thierreich.) 



pores, for the protrusion of the tube-feet. The other five zones, 

 the inter-ambulacral zones or areas (int. ami'), have the plates not 

 perforated. At its anal end each area, ambulacra! or inter- 

 ambulacral, ends in a single apical plate, so that the periproct is 

 surrounded by a ring of ten plates, 

 the apical system of plates (Fig. 

 295). Of these, the five that are 

 situated at the ends of the am- 

 bulacral areas are termed the 

 ocular plates (oc), owing to the fact 

 that each of them bears a rudi- 

 mentary eye ; while the five oppo- 

 site the inter-ambulacral areas, are 

 termed the genital plates (gen), each 

 of them being perforated by an 

 opening which is the aperture of 

 one of the five genital ducts, the 

 ducts of the ovaries or testes as 

 the case may be. One of these 

 genital plates (inadr) has a swollen 



and spongy appearance, which distinguishes it from the others : 

 this is the madreporite, through which, as in the case of the 

 structure of the same name in the Starfishes, the madreporic 



inZamb, 



FIG. 295. Apical system <>f plates ami 

 aboral extremities of zones of the shell 

 of a Sea-urchin : ii//. ambulacra! 

 zones; gen. genital plates: int. muli. 

 inter-ambulacral zones ; mttJi: madru- 

 porite; oc. ocular plates ; /, </>/. peri- 

 proct. (After Leuckart.) 



