viii ECHINODERMATA MORPHOLOGY OF SKELETON 323 



altogether disappeared. In some more recent species of the genus Holectypus 

 the genital pore of the posterior basal plate reappears secondarily. The space in 

 the apical system, vacated by the anal area, is occupied by the madrepore (the 

 right anterior basal plate), which greatly increases in size, or else all the five basal 

 plates shift together towards the apical pole, the pores of the stone canal being then 

 distributed over several or all of them. The plates of the apical system may fuse 

 to a greater or less extent. 



b. Clypeastroida (Figs. 277 and 278). The whole apical system is here extra- 

 ordinarily reduced in extent ; it is, indeed, very minute. All the five basals are fused 

 together, and sometimes also fused with the radials. At least four genital pores are 

 retained. Where there are only four, it is always the posterior which is wanting. 

 The pores of the stone canal open in very various ways in the region of the fused 

 basals. Many scattered pores are sometimes found, or one single large pore, or the 

 pores open into irregular pits or grooves. In the family of the Clypeastridat the 



. qo 



FIG. . 27S. Laganum depressum. Less. Apical FIG. 279. Apical system and neigh - 



system and neighbouring parts of the perisome (after bouring parts of the perisome of Meoma 



Loven). rp, Ocellar pores in the fused radials ; mp, ventricosa, Lamk. (after Loven). For 



pores of the uiadreporite in a branched furrow. For lettering see p. 317. 

 lettering see p. 317. 



genital pores have wandered out of the apical system ; they lie either at its edge, or 

 further removed in the sutures between the (paired) rows of interradial plates. 



c. Spatangoida. The apical system of these exocyclic Echinoids is much reduced 

 in extent, although not so much so as that of the Clypeastroida. It varies much in 

 detail, and in a few extreme forms (e.g. Pourtalesia) the primitive condition is to a 

 very great extent obliterated and destroyed. 



1. In many geologically ancient forms the influence of the wandering of the anus 

 out of the apical system is seen in the disappearance of the posterior basal plate 

 (together with the genital pore belonging to it), and in the absence of a central plate. 

 The other basals and the radials (each of which has its pore) have all shifted, 

 and occupy an area which is sometimes circular or almost regularly pentagonal, 

 sometimes lengthened in the direction of the plane of symmetry. In the latter case 

 (Fig. 283). the two middle radials touch in the median line, separating the two 

 anterior from the two posterior basals. The apertures of the stone canal are found 

 in the right anterior basal, which is occasionally somewhat enlarged. 



2. In most recent fossil forms and in the great majority of the extant Spatangoida, 



