Introduction to Animal Morphology. 139 



plate above. As new plates are added at the vertex, pressure 

 increases downwards, and resistance is afforded by the cal- 

 careous arches (auricles) to which the jaws are attached, and 

 which extend over the ambulacral, as well as the interambu- 

 lacral areas. Thus all the peristomial plates fuse as a solid 

 ring round the buccal membrane. In Cidaridce alone all the 

 primary ambulacral plates are entire, and the sutures between 

 them never become effaced by coalescence, and as the bases 

 of the auricles are interambulacral, no opposition is offered 

 to the downward motion of the ambulacral plates, which, being- 

 pushed over the buccal membrane, form a set of overlapping 

 laminse. 



Around the anus is an apical circlet of five pen- 

 tagonal shields (genital plates (Fig. 20, c), large and 

 stellate in Astropyga), each pierced by a genital duct ; 

 with one of these the madreporiform plate is con- 

 fluent, giving it a swollen, rough appearance. Outside 

 and between these genital plates are five smaller, 

 heart-shaped, ocular plates (Fig. 20, ^), each at the end 

 of the ambulacral radius, and bearing on its surface a 

 red, cellular, photoscopic eye, with no refracting 

 media, but with the last twig of the radial nerve end- 

 ing in it. The genital plates are in two pairs in front 

 and behind, and one posterior azygous plate. 



In Spatangidae, the place of Y\p. 20. 



this latter is taken by the madre- 

 poric plate, which, never sepa- 

 rated by a suture from the right 

 anterior genital plate, sometimes 

 (Meoma, Brissopsis) spreads more 

 or less over it, or even (Schizaster) 

 occupies it to such an extent that 

 both genital pore and gland have .t^^^^t^^^^Tl^.^^ft, 

 disappeared. Further extension >°terambuiacrai plates. 



of the madreporic area causes obsolescence of the left 



