324 



COMPARA TIVE ANA TUM } ' 



CHAP. 



when their development is also taken into account, we find the following condi- 

 tions. The posterior basal plate again appears, but never has a genital pore. The 

 central plate also reappears. The apertures of the stone canal spread out from the 

 right anterior basal towards the centre, i.e. on to the central plate. From this they 



FIG. 280. Apical system FK.. L'SI. Apical system of 



of Abatus cavernosus, Spatangus purpureus, rii nun. in 



Phil, (after Loven). For -!/ (after Loven). For lettering 



lettering see p. 317. > ]>. :>17. 



pass on to^ the posterior basal plate, and the three plates fuse together. The sutures 

 between them disappear, and so a large central madreporic plate is formed, which 

 in very many forms shows a tendency to increase in size and to spread out in the 

 direction of the posterior interradius, and thus more or less to press asunder the two 



tiv 



FIG. 282. Apical system and neighbour- 

 ing parts of the perisome of Micraster coran- 

 guinum (after Loven). For lettering see p. 317. 



FIG. 283. Apical system and neigh- 

 bouring parts ol the perisome of 

 Holaster suborbicularis, Defr. (after 

 Loven). For lettering see p. 317. 



posterior radials (Figs. 279-281). The genital aperture on the right anterior basal 

 may disappear, in which case only three genital pores remain. In isolated cases, 

 the left anterior basal plate may also lose its genital pore. 



3. A method of dissolution of the apical system, unique among the Echinoidae, 

 is found in many Collyritidce (Fig. 284). If we imagine that the elongated apical 

 system, described under heading 1 (Fig. 283), becomes very much more elongated 

 in the direction of the plane of symmetry, and breaks into two groups, one anterior 

 and the other posterior, we have the condition in these animals. The anterior 

 group contains the four basals, of Avhich the right anterior is the madreporitic 

 plate, and three radials, viz. the anterior unpaired, and the right and left anterior. 



