DEVELOPMENT OF ANTEDON (COMATULA, LAMK.) EOSACEUS. 739 



adjacent radial (Plate XLI. fig. 5) ; and a continuity is thus established between the 

 several parts of this canal-system, not only radially but peripherally. At the some- 

 what later period represented in Plate XLII. figs. 6, 7, the channels are completely 

 covered in, so as to be converted into canals ; and each embouchure is divided by a small 

 calcareous islet into two passages, one of them opening opposite the canal of the basal, 

 and the other opposite the corresponding canal of the adjacent radial. In this manner 

 are formed the canals already described in the first radials of the adult Antedon (§ 34). 

 The upper margin of the first radial now shows on either side of its centre an elevated 

 articular surface (Plate XLI. fig. 2, r', a, a), the calcareous reticulation of which is much 

 closer than that of the rest of the plate ; and each of these gives attachment along its 

 dorsal border to a distinctly fibrous ligament connecting it with the corresponding arti- 

 cular surface of the Second lladials; whilst from the ridges which form its ventral 

 border there are now seen to pass towards the opposite face of the second radials a set 

 of larger and more defined parallel fibres, which, from their similarity to those occupying 

 the like position in the adult, we know to be muscular. 



78. The Second Radials (Plate XLI. fig. 2, r^) now begin to show a modification of 

 their original nearly cylindrical form ; being somewhat widened out at their lower end, 

 so as to form two articular surfaces [a, «), which are not, however, du-ectly opposite to 

 those of the first radials, but slope away from them ; so that whilst in close proximity 

 with them on the dorsal face, where they are connected by ligament, they are separated 

 by a considerable interval on the ventral side where muscle intervenes, — an aiTangement 

 which gives to the muscular fibres the length required for theu" efficient contraction. 

 The upper end {h) of the second radial, however, is simply rounded ofi", presenting no 

 proper articular surface ; and it is connected with the base of the Third Eadial by liga- 

 ment only. Although the dorsal surface of the second radial is everywhere convex, its 

 ventral face still shows a deep groove along its median line ; this, however, is separated 

 by a distinct layer of calcareous network from the axial canal beneath it, which opens 

 at its lower end between the articular surfaces, and at its upper in the centre of the 

 rounded termination. 



79. The Third or Axillary Radials (Plate XLI. fig. 2, r') also show but a slight 

 departure from the original simple type ; their lower portion being incompletely cylin- 

 drical, whilst the upper is somewhat expanded laterally to form a pair of articular 

 surfaces, which look obliquely towards either side, and are separated by a projecting 

 median ridge. The lower extremity is rounded off, so as to resemble the upper end of 

 the Second Radial, and shows, like it, the orifice of the axial canal in its centre ; whilst 

 each of the articular surfaces at its upper end shows a similar perforation, which is the 

 orifice of one of the two branches into which the axial canal bifurcates in the interior 

 of the segment, for the supply of the two Arms borne by it. The dorsal face of this 

 segment is everywhere irregularly convex ; but the ventral face has a deep depression in 

 its centre, the bottom of which almost reaches the axial canal. 



80. The segments which form the skeleton of the Arms (Plate XLII. fig. 5) at this 



