254 THE VOYAGE OF H.M.S. CHALLENGER. 



many other genera. There are iisnally three to each side arm plate, and they grow 

 thicker as they near the l)ase of the arm, where they acquire the form of little, blunt, 

 rough spines (figs. 10, 11, 12, q). Besides double tentacle hooks, there are others that 

 are simple, and, from the grain on which they are mounted as a base, may be termed 

 grain hooks. Those that first appear are simple spicules, bent or straight, standing on 

 the side arm plates, above the tentacle hooks (figs. 8, 9). Then a granule is formed 

 under them (fig. 13). More of such hooks grow on the grains or little swollen plates 

 which occupy the position of ujjper arm plates among Ophiurans (fig. 10). Later 

 there remain on the side arm plate only the true tentacle hooks, while the grain hooks 

 stand on those double rows of raised grains which give the ringed or burred look to the 

 small branches of Astrophytons (PI. XXXV. fig. 19). As they approach the disk and 

 thicker part of the arm these raised rows sink and their hooks disapj^ear, and a coarse 

 granulation overgrows the first layer of swelled plates, so that the surface of tlie arm 

 becomes even. The side arm plates which began as ridges encircling the whole arm 

 change their character rapidly. In the central depression between them, on the upper 

 side of arm, a little upper arm plate begins to form (PI. XXXVI. fig. 5), like a perforated 

 lime crust. Then, as the arm enlarges, the side plates separate above, and between them 

 are formed additional scales, which occupy the position of upper arm plates, but follow 

 no rule in their growth (fig. 7). They do not even multiply by the irregular method of 

 Hemieuryale pushdata} These scales, at first thin (figs. 7, 15), afterwards thicken and 

 become more rounded (figs. 10, 11), and some of them make the basis of the two annular 

 rings of grains carrying the grain hooks, which afterwards drop off", so that at the base of 

 an arm there appear (in a dry specimen) only the thickened skin, with a granular coat 

 and a few irregular plates above the side arm plates. These last, early separated above 

 (figs. 7, 15), maintain their union underneath (figs. 6, 12, 16, 17, 19, i). It follows that 

 the growing arm rises more and more above them. They retain their simple form almost 

 throughout, but, within the disk, in fuUy grown specimens they are broken in two 

 (fig. 19, /). The under arm plates first appear about two forks from the tip of the arm ; 

 not, however, simple, but divided in three parts (fig. 12, h), which may still be seen 

 inside the disk of young specimens (fig. 17, li, k).'^ In adults these plates, at the third 

 fork of the arm, are in four triangular pieces, making together an oblong figure. Within 

 the disk the number of pieces is considerable and their form irregular (fig. 19). In 

 this respect there is a marked difference from Ophiurans, whose upper arm plates may 

 be composed of several pieces developed under certain rules, but whose side and under 

 plates are almost always simple, rarely of two pieces, and in one species only 

 [Ophiomyxa 2'>ciitagona) of three pieces. 



1 Bull. Mus. Comp. ZooL, vol. iii., part 10, pi. v. figs. 8-11. 



* Lutken's figures indicate that the young of Gor(jonocephahis eucnemis has the under arm plate not divided 

 (Addit. ad Hist. Oph., vol. i., pi. ii. figs. 176 and 176'). 



