88 



skeleton of the disc in the Brisinga really forms a connected whole, a solid calcareous 

 frame, to the exterior side of which the skeleton of the arms is attached. It has been 

 however shewn that this frame is actually composed of the same elements as the arabu- 

 lacral skeleton of the arms, namely of 2 sets of ambulacral vertebrae, in a line with the 

 series of vertebrae in the arms, and properly representing the interior continuation of these 

 series. It has likewise been shewn that the exterior set of vertebrae in very young speci- 

 mens, come out of their connexion with the oral ring, and thereby evidently connect them- 

 selves immediately with the skeleton of the arm. Properly therefore there remains in the 

 skeleton of the disc only a single set of vertebrae, connected with each other by interposed 

 calcareous plates, so as to form a connected ring which surrounds the oral aperture. 



If we now examine how the case stands in other star-fishes, we shall also find that 

 the proper ambulacral skeleton of the disc forms here only the immediate continuation of 

 the series of vertebra of the arms. Only the interior set of vertebraB are in immediate 

 contact, or form a closed calcareous ring round the oral aperture; while the other part of 

 the series of vertebra?, reckoned as belonging to the disc, is only indirectly connected by 

 the skin of the disc and by the calcareous plates imbedded in the same. Accordingly as 

 the arms are longer or shorter, the number of vertebrae entering into the composition of 

 the disc is extremely variable in the various star-fishes. In certain forms (Goniaster, Cul- 

 cita) where the arms are only indicated by obtuse angles, the whole ambulacral skeleton 

 may properly be said to be absorbed in the disc; in the long-armed star-fishes, for instance 

 Asterias, the part of the ambulacral skeleton belonging to the disc is, on the other hand, 

 only insignificant; while the greater part belongs to the arms. Between these extreme 

 points, we find again all possible transitions. It is now easy to see that the skeleton of 

 the disc in the Brisinga can by no means be compared with the whole skeleton of the disc 

 in other star-fishes, but only with its innermost part (the interior or the 2 interior sets of 

 vertebrae which immediately surround the mouth). The apparently very anomalous structure 

 of the skeleton of the disc in the Brisinga, will then be found in complete conformity with 

 what we know of other star-fishes; only with the difference that the interior vertebrae are 

 necessarily more firmly and intimately connected together, in order to support the enor- 

 mously developed arms. In Tab. V, fig. 1 & 2 there is represented the oral ring of a 9-armed 

 specimen of Brisinga coronata, with the basis of the skeleton of .3 of the arms in natural 

 connection with the disc; and for the sake of comparison, there is also represented in 

 fig. 7 & 8 a corresponding part of the ambulacral skeleton of a likewise 'J-armed spe- 

 cimen of Solaster endeca. We shall here recognise homologous parts ever}'\vhere. As in 

 the Brisinga, so also in the Solaster and other star-fishes, the interior set of vertebras 

 acquire a peculiar development; the vertebrae connecting themselves with each other so as 

 to form a sort of oral ring. The connexion takes place here also by means of peculiar 

 connecting plates perfectly corresponding with the 2 sorts of interposed plates in the oral 

 ring of the Brisinga. The wedge-plates especially are plainly developed, without however 



