584 



ECHINODERMATA CRINOIDEA 



mouth, from which radial cords are given off which run under 

 the ambulacral grooves (Fig. 266, nerv.rad.v). 



The fibres of this nervous system are, as in Asteroidea, 

 immediately beneath the bases of the ectoderm cells. A large 

 band of fibres is given off to each podium, which is covered with 

 minute elevations, each with pointed sense-hairs in the centre. 

 As the animal grows, another nervous system makes its appear- 

 ance, which is developed from the coelomic wall, the cells of 



Fig. 266. — Diagrammatic longitudinal section through one arm and the opposite inter- 

 radius of Antedon, ax, Central canal of centro-dorsal, with prolongation of genital 

 stolon ; B, rosette, consisting of coalesced basals ; Br^, Br^, Br^, Br*, the first four 

 brachial ossicles; chamh, chambered organ ; coel.coe, coelomic canal of arm; gen.coe, 

 genital canal of arm ; gen.r, genital rachis ; gen.st, genital stolon ; m.p, madreporic 

 pores ; musc.long, longitudinal muscle ; nerv.rad.d, dorsal radial nerve ; nerv.rad.v, 

 ventral radial nerve ; p.c, pore-canal ; pod, podium ; H^-R^, 1st to 3rd radials ; 

 st.c, stone-canal ; sub.coe, sub-tentacular canal of arm ; jr.v.r, radial water-vessel. 



certain tracts of which multiply and bud off ganglion cells from 

 which the fibres grow out. 



This " aboral nervous system," as it is called, has its centre in 

 the "chambered organ" (Fig. 2 66, c/m?;i&), which is embedded in 

 the centro-dorsal ossicle, and is roofed over by a plate called the 

 " rosette." This represents the five coalesced " basals," a ring of 

 plates which in other forms alternate with the lowest radials, 

 and it intervenes between these and the centro-dorsal. The 

 chambered organ consists of a ring of five vesicles, which 

 have originated as pouches of the aboral coelom (Fig. 266, 

 chamh). The walls of these vesicles develop nervous matter ; 

 from them radiate out five great cords, deeply embedded in the 



