218 



PHYLUM ECHINODERMATA. 



IV 



FIG. 159. Clypeaster rosaceus L. apical system and adjacent 

 parts (from Delage after Loven). The radials and basals are 

 fused to form the single plate pa covered with pores, at the 

 apical pole, and the genital openings gtx have shifted into 

 the interambulacra, I-V ambulacra numbered. 



the radial and basal plates all fuse to form a single plate at 

 the apical pole on which the madreporitic pores are distri- 

 buted with very 

 various arrange- 

 ment (Clypeast- 

 roids, Fig. 159). 

 Further, the mad- 

 reporitic pores, 

 though typically 

 connected with 

 the right anterior 

 basal, often ex- 

 tend beyond it on 

 to the posterior 

 basal (Fig. 160) or 

 even on to all the 

 basals, and in rare 

 cases beyond the 

 apical system on the adjacent interambulacral plates. 



Lastly in some Clypeastroids the genital openings are found 

 beyond the apical system. In Clypeaster itself (Fig. 159) they 

 lie at some little distance from the basals in the interambulacra, 

 and sometimes they are found be- 

 tween the basals and the first inter- 

 ambulacral plates. 



It would appear that the association of 

 the genital and water pores with the basal 

 plates is not necessarily a fundamental fea- 

 ture of Echinoid anatomy. In the case of 

 the genital pores this association is un- 

 doubtedly not an essential feature, for not 

 only are these pores in some forms dis- 

 sociated from the basals, as we have just 

 seen, but in the young of all forms which 

 have been examined the basals are not so 

 perforated. The water-pore likewise is some- 

 times dissociated from the basals in the 

 adult, and in development appears before 

 them. S. Loven* puts the matter in this 

 way. "The madreporite is not an integral 

 part of the calycinal system, but an extra- 

 neous accessory, and there exists no such thing as a madreporic plate." 



In most Holectypoida the genital aperture of the posterior basal is 



* Monograph on Pour tale sia, loc. cit., p. 76. 



llf 



Fio. 160. Apical system and ad- 

 jacent parts of young Spatan- 

 gus purpureus showing ethmo- 

 lysian condition (from Delage 

 a'fter Loven). r radials ; b 

 basals ; mdp plate with madre- 

 poritic pores formed by fusion 

 of right anterior basal, central 

 plate, and posterior basal ; 

 gtx genital pore ; I-V radii 

 numbered. 



