10 



conspicuous at the edges. By these slits the whole dorsal ridge is divided into a series of 

 successive sections, which are cylindrical or somewhat enlarged at both extremities, and have 

 the appearance of corpora in the spine of a vertebrated animal. From the middle of each 

 of these sections there issues on each side — wonderfully like the transverse processes on 

 a vertebra — one of the above mentioned partitions which separate the ambulacral pores 

 from each other. The flattened enlarged extremity of these partitions, which thus represent 

 the lateral parts of the ambulacral plates, rests on the thickened lateral margin of the skele- 

 ton of the arm always at the junction of the adambulacral plates of 2 vertebrae; so that 

 only a single adambulacral plate lies between them. The above-mentioned ambulacral pores 

 will thus be bounded outwardly only by one single adambulacral plate, and otherwise by the 

 ambulacral plates of 2 successive vertebrae. As regards the form of the adambulacral plates, 

 it is on the whole cylindrical; but the interior surface, which makes the lateral boundary 

 of the apertures for the suction feet, is rather strongly concaved. They are separated from 

 each other by distinct inclined slits, which are covered with elastic fibres, and which are, 

 especially at the extremity of the skeleton of the arm, very wide. 



As already noticed, the 2 interior vertebrae of the arm are distinguished by a some- 

 what shorter and broader form of the dorsal part of the ambulacral plates, and by each of 

 them having, immediately above the adambulacral plates, a distinctly developed dorsal mar- 

 ginal plate (see fig. 17). On the interior vertebra of the arm this dorsal marginal plate (c 1 ) 

 is largest, of trapezoidal form, tapering outwards and having a rather large semilunar arti- 

 culating surface, whereby it is articulated with a corresponding marginal plate ou the outer 

 side of the disc; at the aboral extremity it is firmly connected (often quite grown together) 

 with the next marginal plate (c 3 ) which is considerably narrower, aud of an obtusely conical 

 form. These 2 pairs of marginal plates, which are also very distinctly apparent on the 

 exterior of the arm (see Tab. 1, fig. 12 b, Tab 2, fig. 1 & 2) are certainly intended to effect 

 a tinner connexion between the disc and the enormously developed arms, than would other- 

 wise be possible, if the connexion only existed between the ambulacral and the adambulacral 

 plates. 



The skeleton of the arm has moreover in its whole length a tolerably uniform 

 appearance, excepting only that it tapers gradually towards the extremity, whereby also the 

 individual vertebrae are successively reduced in size. The extreme point of the skeleton of the 

 arm exhibits however, when more closely examined, a very abnormal constitution (fig. 21 & 22); 

 the single vertebrae being here fused together in a rather large convex plate, which serves 

 as a protection for a peculiar organ of sense afterwards described. 



