258 BULLETIN OF THE 



illustrated by the arm-plates and spines. If we examine the broken end of 

 an arm which is repairing, and where new joints arc rapidly forming, we shall 

 see that the tip is a mere tube (Plate V., Figs. 1, 2, 3, 4. Compare, also, 

 •the figures of young Ophiurans given by Miiller). This tube is a calcareous 

 network, filled and covered by the secreting tissue, or, as it may be termed, 

 the skin. There seems, then, to be no beginning of an internal arm-bone, — 

 nothing hut this open calcareous tube. Immediately, however, there appear 

 annular strictures round the arm, marking the future joints, with a sunken 

 longitudinal line, or even s'it, above and below, dividing the tube into two 

 side arm-plates. This embryonic stage is partly persistent in some genera ; 

 e. g. species of Ophiomusium, which have no under arm-plates on most of 

 the joints. Then appear on the central point of juncture, above, clusters of 

 grains, which, in time, grow into upper arm-plates, and a similar process fol- 

 lows for the lower arm-plates. On the lower surface may lie seen (Fig. 1), 

 on the terminal joints, a little flap on each side ; this flap grows more acute 

 and rounded, becomes separated from the side arm-plate, and ends as a true 

 arm-spine. So that, in this species of Pcctiiutra, beginning with a tube 

 of calcareous network, covered by its secreting skin, we end, at the base of 

 the arm, by the complex assemblage shown in figures 5, G, 7 ; and all these 

 parts are merely different growths and divisions of this same network. 



The same is true of the various divisions of the upper arm-plates and their 

 supplementary pieces, explained in Plate V. The network may send its 

 branches from its edges or from its upper surface, and these branches ma) 

 remain connected, and thus enlarge the plate; or they may be separated, and 

 make spines, tentacle-scales, and supplementary plates. It is the same with 

 other Eehinodermata, and the process is simply illustrated by the growth of 

 a young spine in Ophiothr'uc (PI. III., Figs. 4-7). The mouth-parts make no 

 exception. The jaw-plate (PI. I., Fi'_ r . 4, e) is a skin-plate, and supports 

 another skin-plate, the tooth ('/"), which has been separated from it. The 

 peculiar papilla; of Ophior/li/pJia, which embrace the second mouth-tentacle 

 (7"), are, on one side, carried by a peculiar piece attached to the innermost 

 under arm-plate. This piece is only an enlarged outer mouth-papilla (so 

 called), on whose edge have formed these additional papilhe (sec the mouth- 

 papilhe described under Pectinura marmoratci). A similar process gives all 

 the variety in the lamelhe of the stony Polyps. "What is true of the arm- 

 plates is true of the skin of the disk, whose scales may lie traced from the six 

 primary plates first formed on the hack. On these scales may be developed 

 spines grains, or stumps, just as on the arm-plates. The strict morphologi- 

 cal connection of all these parts should warn us not to distinguish them too 

 emphatically, and not to give them too great a value in generic distinctions, 

 especially those minute papilla; which form the armature of the mouth. As 

 to what is provisionally called the skeleton, it is well to remark that Gaudry 



