H. L. Hawkins — Apical System in the Holectijpoida. 15 



H. The TitEND of Evolution of the System. 



It seems advisable to explain that the evolution indicated here is 

 not necessarily coincident with the phylogeny of the group, but 

 applies only to the lines of development of the one structure. 



The passage of the periproct out of the apical system resulted at 

 first in the complete destruction of the posterior genital and a displace- 

 ment of the remaining plates into a transverse rather than a radiating 

 series. This is the stage represented by the ' Plesiechinus ' section of 

 Pygaster (PI. II, Fig. 1). Subsequent stages may be divided into two 

 series : first, those in which the posterior genital was permanently 

 absent, and second, those in which it reappeared. 



The simplest condition of the first series is exemplified by Anortlio- 

 pygus (PL II, Figs. 3 and 4). Here, after the complete separation of 

 tlie periproct from the apex, the madreporic genital passed back 

 through the system, which therefore becomes ethmolysian. This 

 condition results in an apical system hardly differing from that of 

 a Spatangid, as may be seen by comparing Figs. 4 and 11 of 

 Plate II. 



By a slightly different process the two posterior oculars, instead of 

 being widely separated by the intrusion of the madreporite, became 

 enlarged and met one another across the posterior margin. The two 

 postero-lateral genitals followed them, and later the two antero-lateral 

 oculars. The various stages in the consequent thrusting forwards of 

 the madreporite are indicated by a comparison of Figs. 4, 8, 9, 13 

 (Plate II), treating them as a sequence. Co7iulus is the only true 

 Holectypoid that shows this condition typically. It is, however, 

 a very general one among Irregular Eehinoids, as its presence in such 

 widelv differing forms as Pyrina, Hemiaster, and Holader shows 

 (cf. PI. II, Figs. 9, 10, and 13). 



The second direction of development commenced with the Pygaster, 

 sens, str., group (PI. II, Fig. 2), where the place of the fifth genital 

 was occupied by one or more small plates distinct from those of the 

 anal membrane. In some forms, notably among the Echinohrissi, the 

 presence of more than one of these posterior plates became common, 

 though not universal. The madreporite is large and tends to occupy 

 all the centre of the system. In Holectypus, sens. str. (PI. II, Fig. 5), 

 the posterior genital became single and imperforate (a condition often 

 present in Bisco'idea), and finally, in Ccenholectypus (PI. II, Fig. 6), 

 it became perforated, thus rendering the apical system complete. 



As a variant from this Holectypus plan, in which the madreporic 

 genital was much larger than the other four, we find the jDisco'idea 

 principle, in which the posterior genital (whether perforated or not) 

 was, in common with the others, of practically the same size as the 

 madreporic plate. The most significant change in this group was, 

 however, the spreading of the madreporic water-pores over all five 

 genitals, accompanied by the reduction in importance of the oculars. 

 The natural outcome of such a process was the apical system of the 

 Clypeastroids (PI. II, Fig. 12), where, apart from the fusion of the 

 genitals, the structure was quite similar to that of Bisco'idea. 



Finally, using the generic names merely to indicate types of 



