ORAL AND APICAL SYSTEMS OF THE ECHINODERiMS. 191 



and the apical systems, although extremely well developed 

 in the larva, undergo very extensive changes which result in 

 the total disappearance of the oral system, and a consider- 

 able modification of the basal element in the calyx. In some 

 tropical species the anambulacral system may reach a high 

 stage of development, but in the British species it is very 

 imperfect even in the larva, and shares the fate of the oral 

 plates by undergoing complete resorption.^ 



I have already endeavoured to determine the homologies 

 of the Crinoidal calyx in the other Echinoderms. Let us 

 now attempt to solve a similar problem with regard to the 

 other elements of their skeleton. The oral system of a 

 Crinoid consists essentially of five plates or series of plates, 

 disposed interradially around the mouth. I have already 

 stated that I have entirely failed to find any traces of these 

 plates in any of Agassiz' figures of Asterid larvae, and that 

 as far as can be judged from Metsclmikoff^ s figures their pre- 

 sence in the Ophiurids is very uncertain.^ In the Holothu- 

 rians, however, the oral plates of the Crinoids are very well 

 represented. Kowalewsky^ figures five large triangular plates 

 around the mouth of the young Psolinus hrevis, without any 

 trace of a commencing skeleton in any other part of the 

 body. He makes no reference to them whatever, but they 

 seem to persist through life ; if not in Psolinus, at any rate 

 in Psoitis ephippifer in which, according toWyville Thomson, 

 a slightly elevated pyramid of five very accurately fitting 

 calcareous valves closes over the oral aperture and the ring 

 of oral tentacles. 



Again, Krolin^' describes a Holothurian larva in which the 

 border of the blunt and rounded anterior end is '^ durch 5 

 vorspringende durchlocherte Kalkscheibchen in eben so viele 

 Lappen getheilt."" Besides these, the whole perisome con- 

 tains a number of overlapping reticulated plates. These 

 also occur in the Cucumaria larva figured by Selenka,^ but 

 there is no trace of orals, and the plates are smaller than in 

 Krohn's larva, and not in contact. 



With regard to the Echini, it might seem at first sight 

 rather a hopeless task to attempt to determine the elements 

 of an oral system among the large number of plates which 



1 W. B. Carpenter, ' Phil. Trans./ loc. cit., p. 471. 



2 See Addenda, No. 5, p. 205 



3 " Beitrage zur Entwickelungs-geschichte der Holthurien," ' St. Peters- 

 burg Memoirs/ tome xi, No. 6, fig. 13. 



* " Beobachtungen aus der Entwickelung der Holothurien und Seeigel/' 

 'Miiller's Archiv./ 1851, p. 347. 



* "Zur Entwickelung der Holotbuiieu/' 'Zeitscbr. fiir Wiss. ZooL/ Band, 

 xxvii, taf. xiii, fig. 28. 



