12 



above referred to are by no means imaginary, but really exist in the form of evident sutures. 

 On the radial spaces these medial sutures may be traced round the whole ring; but in the 

 interradial spaces these sutures are interrupted on the upper side by a narrow single piece, 

 which we will call the wedge-plate (tig. 4, 6, 7, 8, 9 k) just forming the angle at the issue 

 of 2 arms, and the upper tubercular salient extremity of which is always, as before remarked, 

 very distinctly apparent on the exterior of the disc. Besides the sutures mentioned, which 

 thus bisect the radial and interradial spaces of the disc, there may still be observed on 

 closer examination a number of other sutures indicating that the bucal ring is composed of 

 a great .number of single pieces. Of these sutures there are some which go more or less 

 across the ring; others, which have quite a different direction, running along the upper or 

 lower side of the ring or parallel with its exterior or interior edge. The last sutures, which 

 are the most distinctly marked of them all, will be found to lie all in about the same plane, 

 and are especially very conspicuous on the upper side of the radial spaces, and on the lower 

 side of the interradial spaces. The result is that each of these spaces is also transversely 

 divided into 2, or consists of 2 successive sections. If we only consider the sutures running 

 along the ring, we shall thus find that the latter is composed of 2 concentric rings firmly 

 attached to each other; the inner being both higher and wider than the outer. 



If such a bucal ring, with a part j)f the outer tendinous skin still adhering to it, 

 be placed in a concentrated solution of potass, the single pieces connected by suture will 

 after a short time separate completely from each other, and instead of a continuous ring 

 there will appear only a confused heap of numerous single calcareous pieces of various size 

 and form. The bucal ring is thus resolved into its simple elements, the appreciation of 

 which will now present considerable difficulties. 



But if the operation be suspended a little before, it may happen that larger or 

 smaller parts of the bucal ring can be obtained with the single pieces in natural connexion 

 with each other, and yet connected so loosely that one piece after the other may be sepa- 

 rated from the rest with the greatest ease. 



On examining and comparing a number of such parts, it will soon be found easy to 

 get some idea of the apparently extremely complicated structure of the bucal ring, especially 

 on comparing therewith one of the so-called ambulacral vertebrae of the skeleton of the arm. 



If we consider the bucal ring in its natural connexion with the ambulacral skeleton of the 

 arms (see Tab. V, fig. 1 2), we shall easily be able to ascertain that it is chiefly composed of 

 double sets of similar vertebrae to those which form the skeleton of the arms; so that each arm 

 tinued inwards by two ambulacral vertebrae entering into the formation of the bucal ring is con- 

 and therefore immovably connected with each other and with their neighbors. The exterior 

 set of these vertebrae of the disc correspond really in almost every respect with the interior 

 free vertebrae of the arm, and can therefore, morphologically speaking, be more properly 

 said to belong to the arms than to the disc; which is also, as will subsequently be shewn, 

 confirmed by the development. On the other hand, the interior set of the vertebrae exhibit 



