57 



as only one single set of vertebrae enters into the composition of the disc; while the fol- 

 lowing set, which in the adult animal always continues in firm connexion with the oral ring, 

 does here evidently not belong to the ring but to the arms. As regards this innermost set 

 of vertebrae, their form and structure appear here to correspond so perfectly with the proper 

 vertebrae of the arms, that we may really also consider them as belonging to the arms. 

 In this manner there will thus remain only the narrow calcareous ring to which the oral 

 membrane is attached, that can be considered as a special skeleton-formation belonging to 

 the disc: while all the remaining skeleton is only formed by the 10 arms, which are so 

 arranged around a common centre that their interior vertebric are contiguous and grow 

 together with each other. The important inferences which may be hence deduced for a 

 correct appreciation of the nature of the star-fishes, will be noticed more particularly in a 

 subsequent section. 



b. Development of the disc. 



The structure of the disc has been already above noticed in a very early stage of 

 development, whereby it appears evidently enough that the exterior set of vertebra3, which 

 in fully developed specimens enters into the composition of the calcareous ring, does not 

 originally belong to the disc, but represents the interior vertebra; of the arms. In the be- 

 beginning there is also between both these sets of vertebra, a movable muscular connexion 

 similar to that between the other vertebra', of the arms; so that it is in fact only the 

 interior set of vertebra which serves as support for the disc itself. During the subsequent 

 growth and enormous development of the arms, it becomes however necessary that the 

 skeleton of the disc should acquire a greater solidity than could be produced by only a 

 single set of vertebrae; for which cause the interior vertebra; of the arms have to give up 

 their mobility little by little, and, by connecting themselves with the original calcareous ring, 

 to contribute to its more powerful development. 



In quite young specimens with a diameter of disc of 10 — 12""" and a length of arms 

 of 60 — 70""" these vertebrae still retain in some degree their original self-substantiality. They 

 are (see Tab. V, fig. 13 & 15) comparatively to the yet slightly developed interior vertebra?, 

 considerably more prominent, and even project distinctly beyond the periphery of the calca- 

 reous ring, being bounded on each side by a rather deep sinus in the calcareous ring, which 

 indicates the place, where the wedge-plate rises. The connexiou with the interior set of 

 vertebrae has however already acquired so much firmness that these vertebrae, as in adult 

 specimens, always remain attached to the disc when the arms are separated from it. 



With respect to the exterior form of the disc in such young specimens (the smallest 

 having only a disc-diameter of 10°"") it differs from that of the full grown, in the more or 

 less evidently projecting radial parts, as well as in its considerably greater flatness or thin- 

 ness, and in its generally weaker and less compact appearance. It is like the arms, of a 



8 



