1876.] and Development of Antedon rosaceus. 221 



I shall show it to be almost unmistakably established by the history 

 of embryonic development, at a certain stage of which the subdivision 

 of the axial prolongation (ax, fig. 11) into diverging branches, of which 

 one passes to each ray, is very distinctly traceable. 



Arms. 



Each Arm is composed of a linear series of calcareous segments, con- 

 nected with each other by muscles and elastic ligaments, in the manner 

 described in my former Memoir, the muscles being all flexor, whilst the 

 ligaments are extensor. Thus when the muscles (of which a pair inter- 

 venes between each segment and its proximal and distal segments, except 

 where two segments are connected by a syzygy) are called into contrac- 

 tion the arm is coiled up spirally like a watch-spring, whilst when the 

 muscles relax it is straightened out again by the elasticity of the ligaments. 

 — What may be called the centrum of every one of the calcareous segments 

 is perforated by a circular foramen (a a, fig. 6) j and the linear succession 

 of these foramina forms a canal, which is occupied by a cord of proto- 

 plasmic substance (b) given off: from the circlet contained within the first 

 radials, which circlet (as already described) is formed by the lateral 

 inosculation of the five radial cords (I, m, fig. 3) given off from the 

 five-chambered organ (g) contained within the centro-dorsal basin. I 

 have occasionally found, in transverse sections of decalcified arms (espe- 

 cially in Antedon celticus), pairs of branches given off: from this " axial 

 cord " at the junction of the segments to ramify upon the muscles ; and 

 it was this anatomical fact which suggested to me that the " axial cord" 

 has really the function of a nerve, although not possessing its characteristic 

 structure, and that the five-chambered organ must consequently be a 

 nervous centre. That the cord which was supposed by Miiller to be a 

 nerve has an altogether different function, was stated in my previous 

 Paper (§ 19), on the basis of the connexion I had even then demonstrated 

 between that cord and the generative organs developed in the pinnules. 

 And the nerve-function which I then, on anatomical grounds alone, 

 attributed to the axial cord has been since completely borne out by the 

 experiment to be presently cited. 



Along the upper or oral face of the succession of calcareous segments 

 with its intervening muscles there lies a single large canal (cc, fig. 6) 

 occupying the whole breadth of each arm ; this is the coeliac canal already 

 spoken of as an extension of the lower part of the coelom or perivisceral 

 cavity (cc, fig. 12). Enclosed on either side by the perisome, it is shut 

 in at the top by a transverse partition that separates it from the double 

 subtentacular canal (stc, stc, fig. 6) which overlies it. This double canal is 

 virtually single; for the vertical partition which divides it has large 

 irregular openings (fig. 7), which allow a free communication between 

 the two lateral halves *. Where this vertical partition joins the hori- 

 * In some species of Antedon this canal is actually as well as virtually single. 



