THE CRINOIDEA 



125 



(Fig. XXXIII. 5), and finally, in most species of Antedon are 

 resorbed (cf. Fig. XXX. 4). These plates are called oralia (0). 

 Their prominence in early stages shows them to be primary 

 elements of the theca, probably well developed in the adult of 



FIG. XXXIII. 



Development of orals in Antedon. 1 (after Bury), 77? still visible, no RR yet formed. 2 

 (after Allman), closed ; between them and BB are the developing RR. 3 (after Allman), open, 

 exposing oral tentacles ; no arms yet exist. 4 (after W. B. Carpenter), now separated from 

 BB by RR, which support arms. 5 (after W. B. Carpenter), arms cut off above first brachial so 

 as to show O, which now surround the mouth ; the shaded portion represents integument, in 

 which ambulacrals and interambulacrals are developed. 



Note also gradual decrease in size of BB. A further stage in oral history is seen in Fig. 

 XXX. 4, which is from another species. 



primitive forms. Five triangular plates that cover the mouth in 

 the recent Holopus, Hyocrinus, Rhizocrinus, and Thaumatocrinus, 

 are by all writers homologised with orals (Fig. XXXIV.). In 

 all Antedonidae, Bathycrinus, and Calamocrinus, they are almost 



Amb 



iAmb, 



Fio. XXXIV. 



Teginen of Holopus. The arms 

 are removed from the side nearest 

 the observer, showing the articu- 

 lar surfaces of R, the radials, 

 which are fused together. Amb, 

 ambulacrals, which pass down the 

 brachials to the tegmen and a little 

 way up between the orals ; iAmb, 

 interambulacrals, partly separat- 

 ing brachials from orals ; 0, five 

 perforate orals; Rp, processes of 

 fused RR. (After P. H. Carpenter, 

 1888.) x |. 



Fia. XXXV. 



Tegmen of Haplo- 

 crinv.t mespiliformis. 

 Br, first brachial; 0, 

 oral ; p, pore in post. 

 O ; R, radial. (After 

 Wachsmuth & Sprin- 

 ger, 1888.) X 6. 



FIG. XXXVI. 



Tegmen of Hybocriiius coni- 

 cus. Small irregular ambula- 

 crals overlie the apposed edges 

 of 0, the orals, which are 

 shaded. Post. has a hydro- 

 pore, and is therefore a madre- 

 porite, M. As, anus, lies be- 

 tween this and x, the anal 

 plate of the cup. R, radials, 

 with arm -facets shaded, and 

 nerve channel black. (Based 

 partly on MS. drawings by W. 

 R. Billings. Natural size.) 



or entirely resorbed in the adult. The ambulacra pass to the 

 mouth between or below the orals. 



In Hybocrinus, Haplpcrinus, Carabocrinus, and other primitive 

 genera are five interradial plates precisely resembling the orals of 

 the larval Antedon in shape and position (Figs. XXXV., XXXVI.). 



