3-24 



COMPARATIVE AXAT02IY 



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. 2S0.— Apical system 

 of Abatus cavernosus, 

 PMl. (after LovSn). For 

 lettering see p. 317. 



Fig. 2S1. — Apical system of 

 Spatangus purpureus, s:; imn. in 

 size (after Lov6n). For lettering 

 .see p. 317. 



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

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

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

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



Fig. 2S2.— Apical system and neighbour- 

 ing parts of the perlsome of iVIicraster coran- 

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



Fk :. 2S8.— Apical system and neigh- 

 bouring parts ot the perisome of 

 Holaster suborbioularis, Defr. (after 

 Lov^n). 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 Co//>jritiilcc (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 which the right anterior is the madreporitie 

 plate, and three radials, viz. the anterior unpaii'ed, and the right and left anterior. 



