210 



PHYLUM ECHINODERMATA. 



to these may be added the peristomial plates, i.e. the plates in 

 the peristomial membrane in the middle of which the mouth is 

 placed (Fig. 148 b). 



In the regular forms or Endocyclica (Diadematoida, Cidaroida, 

 most Palaeechinoidea), the apical system (Fig. 149) consists of 

 (1) the periproct area containing a number of small plates, the 

 periproct plates, amongst which, towards the right posterior 



radius (No. I) or in- 

 terradius (I. II) lies 

 the amis (an) ; (2) 

 the five interradially 

 placed basals (genitals), 

 which are usually per- 

 forated each by a 

 genital opening (go), 

 and one of which (m) 

 is perforated by the 

 water - pores of the 

 m a d r e p orite. With 

 this system, though 

 not belonging to it, 

 must be mentioned 

 the five radially placed 

 ocular plates (radials) 

 which are perforated 

 for the small terminal 

 tentacles of the water- vascular system : these plates are really 

 the terminals and belong to the ambulacral surface. Some or 

 all of the radials may be wedged in between the basals and 

 assist in forming the boundary of the periproct. 



Occasionally, as a specific or individual character, more than one 

 genital opening is present on each genital plate. Thus two pores have 

 been found in Cidaris perornata, Arbacia punctulata, etc., five on the 

 madreporite of Echinus acutus. In short, among both fossil and recent 

 species there is some variability in this character.* 



In the young of the Saleniidae the centre of the apical system is occupied 

 by a central, on one side of which, in the right posterior interradius, the 

 anus is placed. This condition is repeated in most young sea-urchins 

 (Fig. 150), but in adults a number of small plates the periproct or 



Flo. 149. Apical plates of young Strongylocentrotus droe- 

 bachiensis (from Lang, after Loven), an anus among 

 the periproct plates ; go genital openings on the basal 

 or genital plates ; m madreporite (right anterior 

 basal) ; r radial plates (ocular). 



* In the cretaceous and tertiary genus Goniopygus (Arbaciidae) the 

 genital openings are outside the apical system. 



