ECHINODERMATA 



453 



and serve for the support of the arms (Fig. 224 ri-rm). 

 Through the vigorous growth of the radial pieces, to each 

 one of which two other plates are added, the oral plates 

 are crowded on to the oral surface, where they finally 

 undergo resorption. In other Crinoids (e.g., Uhizocrinus) , 

 on the contrary, the oralia are said to persist throughout 

 life. Another change has taken place at the base of the 

 cap : the centrodorsal plate has gradually overgrown the 

 ha<alia and the lower radialia, so that nothing more remains 

 visible of the chief pieces of the original cup. The basalia 

 have fused into an unpaired piece, the so-called rosette. The 



e. m. r.l). rro. rr. 



Fig. 224 —Vertical axial section through the disc and point of attachment of an 

 arm of Antedon rosacea (with slight changes, after H. Ludwig). At the left the 

 section passes through an interradius, at the right through one of the radii, av, 

 axial body cavity ; br,^ bi-j, Ir^^^ hr^^ brachialia (skeletal pieces) ; cl, circum- 

 visceral body cavity; cr, cirri; d, intestine; de, dorsal canal of the arm; dn, 

 dorsal nerve; do, dorsal organ; e, epithelium of the ambulacral groove; g, 

 genital canal; gek. o, chambered organ; il, intervisceral body cavity; to, mouth- 

 opening ; mil, musculature of the arm ; p, pores of the cup ; r^ r^^ r,„ radialia 

 (skeletal pieces) ; rh, radial blood-vessel ; rn, radial (ventral) nerve ; rw, radial 

 water-vessel; st, stone canals; t, tentacle; vo, ventral canal of the arm; wr, 

 water-vascular ring and the stone canal {st) proceeding from it. 



stem of the pentacrinoid stage now also degenerates. In 

 place of it there are developed from the centrodorsal plate 

 (according to Perkier as evaginations of the chambered 

 organ) at first five and later additional cirri, by means of 

 which the young Rhizocrinus attaches itself (Fig. 224 cr). 



