56 



in connexion with the wedge-plate. Neither can there be certainl}' distinguished any evident 

 madreporic body. The arms issue like immediate prolongations from and round the disc; 

 they are broadest at the base, and thence rapidly and evenly tapering towards the extre- 

 mity which however as before mentioned was broken off in them all. Through the trans- 

 parent dorsal skin of the arms, in which every ti'ace of calcareous keels or transyerse ridges 

 is still wanting, there appears the subjacent ambulacral skeleton with its medial elevated 

 crest formed by the dorsal part of the ambulacral plates. The single vertebro3 are of a 

 particularly narrow and elongated form. Along each side of the arms there is seen a single 

 row of spines, which are all provided with a distinct skin-sheath thickly larded with pedi- 

 cellaries. According to their position, they appear to answer to the outer furrow-spines in 

 the developed individuals. 



If we turn the specimen round, and view it from the ventral side (see fig. 40) the 

 naked skin-like area which occupies nearly the whole of the underside of the disc, with the 

 widely gaping circular oral aperture (o) in the middle of it, strikes the eye immediately. 

 Only in the extreme periphery there appear those calcareous parts which support the disc 

 and the arms (the ambulacral skeleton) which as regards the disc forms an extremely nar- 

 row frame, composed of a single set of vertebrae, whence the skeleton of the arms takes its 

 issue. In conformity herewith we see only a single circle of 20 water-feet belonging to the 

 disc. These water-feet are placed in pairs at the extremities of the ambulacral furrows 

 which run along the ventral side of the arms; each pair separated from the next by an 

 obtusely conical calcareous piece (ad*) projecting inwards from the angle between 2 arms, 

 and formed by the 2 contiguous adambulacral plates belonging to the 2 adjacent vertebra. 

 The ventral furrows of the arms taper rapidly outwards, and are, between each successive 

 pair of water-feet, distinctly instricted. The adambulacral plates which limit the ventral 

 furrows on the sides are long and narrow, yet still indistinctly separated from each other, 

 and only furnished each with a single spine in the middle. Likewise each of the innermost 

 adambulacral plates (ad ') belonging to the disc and joined together in one piece, is fur- 

 nished in the middle with a long spine directed obliquely downwards and outwards. Of the 

 oral spines, spread out fan-like towards the mouth, which in fully developed specimens are 

 so distinct, there is yet no trace to be seen. Immediately before the interior extremity of 

 the ambulacral furrows, and partly covered by the point of the 2 contiguous innermost 

 adambulacral plates, there extends round the whole ventral side of the disc a narrow calca- 

 reous ring, to which the oral membrane (m) is attached. It answers to the circular ridge 

 projecting inwards from the oral ring in adult animals, and consists similarly of 20 single 

 pieces, which, immediately before the ambulacral furrows and the point of the interradlal 

 spaces, are connected with each other by evident sutures. 



It will be seen from the above description that the disc in the young Brisinga men- 

 tioned, although externally appearing to be rather large in proportion to the arms, is yet 

 really, with due regard to the skeleton, stiU more reduced than in the full grown animal; 



