62 



connected with the oral ring, but in the early stage appear only in the form oi 2 thin 

 bone pegs (rj loosely attached to the oral ring. Of the 2 following pairs of marginal 

 plates belonging to the arms there is still, in the young animal above described, not the 

 smallest trace to be seen. 



In specimens with a disc-diameter of 11 — 12 n,m the oral ring (fig. 13) is still only , 

 very slightly developed; although it already consists of all the calcareous pieces which, in 

 fully developed individuals, enter into its composition. It is especially very thin and flat; 

 so that its upper surface is nearly horisontal, and lies about level with the skeleton of 

 the arm; while in full grown individuals it is strongly convex, and towards the interior 

 considerably raised above the skeleton of the arm. This comes mainly from the slight deve- 

 lopment of the interior set of vertebra?, which still retain much of their original appearance 

 similar to that of the arm-vertebrae. It has been before mentioned that the exterior set of 

 vertebra?, which likewise belong to the oral ring, and which in full grown individuals are 

 considerably reduced in size, correspond in the early stage completely with the arm-vertebrae 

 botli in size and form, as well as by their looser connexion with the interior set of vertebra', 

 being moreover distinctly salient in the periphery of the oral ring. In consequence of the 

 peculiar form of the oral ring hereby produced, the wedge-plates (fig. 14. 15 w), which in 

 full grown specimens are more or less vertical, lie here in almost perfectly horizontal posi- 

 tion; and on each side of them there appear, on the upper surface, the 2 pairs of ambu- 

 lacral pores clearly and distinctly limited, the one behind the other. 



e. Development of the cuticular skeleton. 



The development of the calcareous particles enclosed in the dorsal skin proceeds, 

 exactly in the same manner as the development of the ambulacral skeleton, by a radiary 

 ramification of originally simple isolated calcareous granules. In the dorsal cuticle of the 

 disc, this development of the calcareous pieces to which the disc-spines are attached, is 

 easily observed in quite young specimens, in which these calcareous pieces have still parti- 

 ally the form of small thin perforated circular discs. In the young specimen above described 

 there is in the dorsal cuticle a more continuous extraordinarily thin calcareous net with wide 

 circular meshes (Tab. VI, fig. 33), evidently a relict from the embryonic state, and which is 

 afterwards replaced by the usual limited calcareous parts corresponding with the spines. 

 Just opposite to the angles between the bases of the arms there can however be discerned 

 10 somewhat more distinctly limited plates, but of the same fine reticular structure. These 

 plates (see Tab. IV, fig. 39) belong here most undoubtedly to the skin, but enter subse- 

 quently into connexion with the rudimentary wedge-plates, and form the tubercular promi- 

 nence afterwards so conspicuous on the same. 



The dorsal transverse ribs are indeed not outwardly visible until after that the arm 

 has already attained a considerably development; but their foundation is laid very early, 



