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



Fig. 12. y. Lower side of fig. 11, showing the triple under arm plate (h). 



„ 13. ^§°- ^ grain of the arm, from a terminal twig, bearing a rudimentary hook, 

 which has not yet begun to bend, and exhibits its building by ii-regular lime 

 spicules. 



,, 14. If. A tentacle hook from near the tip of a twig, in profile, showing the holes 

 of growth, which indicate its formation by two parallel spiculse. (See also 

 Bull. Mus. Comp. Zool., vol. iii. part 10, pi. iii. fig. 10.) 



„ 15. \°. Last fork of the arm, from above. The delicate, tuberculated scaling is a 

 further growth of that shown in fig. 7. One tentacle hook may be seen on 

 each side arm plate. 



„ 16. */*. The same fork, from below, showing the large side arm plates meeting on 

 the median line. 



,, 17. \2. Two angles of the disk of a very young specimen, from below. The plates 

 of the interbrachial space (see fig. 2), as well as those of the back, are covered 

 by a granulation, which does not, however, hide the genital plate and scale 

 (o, n) nor the single madreporic shield (a). The large side arm plates (i) 

 carry tentacle scales, which have changed from the form of hooks (figs. 10) 

 to that of rough-ended spines the triple under arm plates {h, h); the first 

 one is, however, single ; side mouth shield (h), jaw (c), jaw plate (e), 

 tooth papillae {d'). 



,, 18. 2fl. The skeleton of two mouth angles, from above, and of the intervening arm 

 with the genital plates (o). Between their inner ends begin the mouth 

 frames {/), outside which are two arm bones ; peristomial plates (v), divided 

 in halves; a division more or less observable also in the adult, and difi'ering 

 from the single bone in Euryale aspera (PI. XXXV. fig. 2, v). The 

 small angle piece sometimes considered the first under arm plate (y'), jaw (c), 

 jaw plate (c), tooth papillae in a single row {d'), mouth tentacles of the 

 first and second pairs, dried and shrivelled (?•', r"). 



,, 19. f . Mouth angle of an adult, with the base of an arm, and the madreporic shield 

 (a), side mouth shield {b), jaw (c), jaw plate (e), tooth j^apillas {d'), 

 side arm plate divided in two (^), tentacle pore (r). The rest of the mosaic on 

 the lower side of the arm is the broken under arm plates. 



PLATE XXXVIL 



Plates XXXVIL-XLIIL represent the internal hard parts, or skeletons, of Ophiurans. 

 Their treatment is nearly uniform. One brachial and portions of the neighbouring 

 interbrachial spaces are presented ; and that part of the roof of the disk is shown as if 



