REPORT ON THE OPHIUROIDEA. 253 



No distinct radial shields are yet \'isible ; but along the margin of the disk are over- 

 lapping plates, which, increased in number and size, may be found under the skin of the 

 adult. A ring, consisting of two large side arm plates {occupying the under surface) and 

 four pieces representing upper arm plates, surrounds the arm (hg. 2). Fig. 1 gives a 

 lower view of the entii-e animal of figs. 2 and 3 before drying. All the lower plates are 

 concealed by the thick skin, although the upper ones may be distinguished. There are, 

 as yet, but two arm forks, and the first one is far from the disk, as in Trichaster. There 

 is one mach-eporic plate in the usual position of a mouth shield. It looks like a little 

 pimple, but has been omitted by the HthogTapher. Tentacle scales, like little hooks, are 

 found as far as the joint where the arm joins the disk. By the time the disk has attained 

 a diameter of 7 mm. considerable changes have taken place. The granulations, which 

 had only appeared as lines in the younger stage, now almost wholly hide the plates, both 

 above and below (fig. 17). The genital plates and scales {o,n) not noticed before are 

 now prominent. The madi-eporic shield (a) is swollen and perforated ; and the jaw has 

 a well-marked jaw plate, and on the sides mouth papHl^e. In a fully grown specimen 

 traces of the young stage may still be followed. On removing the skin from the mouth 

 angles of a disk 60 mm. in diameter (fig. 19) there appear a jaw and jaw plate (c,e) more 

 rounded and less elongated than in the young. Outside these, but of comparatively 

 small size, are the side mouth shields (h), and, again outside these, the madreporic shield 

 (o). The copious granidation, which, diuing the middle stage, covered the lower inter- 

 brachial space (fig 17) has essentially disappeared, as have the disk plates, which ceased 

 to grow and were obhterated in the thick skin. Above, the disk shows no granidation 

 {Gorgonocephalus agassizii) save in form of a few small spines whose bases are sur- 

 rounded by grains ; just at the margin may be recognised the lines of plates abeady 

 referred to. The chief features of the roof of the disk are the hiarh and lone radial 

 shields, so characteristic of the group. 



Thus, a disk, flat at the beginning and covered with plates quite as among ordinary 

 Ophiurans, proceeds to change, fii-st by covering itself -R-ith a close granulation ; secondly, 

 by the disappearance or atrophy not only of this granulation but of the disk plates, 

 except those of the margin which continue to grow and multiply ; thirdly, by the great 

 development in length and height of the radial shields. 



The beginning of an arm, as illustrated at its tip, difiers in no essential from that of 

 Ophiurans.^ A small swelling or knob makes the end, and indicates the beginning of the 

 next new joint (figs. 4, 5). The penultimate joint is divided lengthwise, above and below, 

 making the side arm plates (i) which enclose the arm. From the outer edge of these plates 

 springs a slender projection of lime spicules which, b}^ a constriction near its base, becomes 

 a small jointed spine. This bends at its point, throws out an additional curved branch, 

 and becomes a double tentacle hook (fig. 14), homologous with those in Ophiothrix and 



1 Bull. Mus. Comp. ZooL, vol. iii., part 10, pi. v. figs. 2, S, 12. 



(ZOOL. CHALL. EXP. PART XIV. 1882.) 33 



