120 ON THE ANATOMY OF OPHIOTHRIX VARIABILIS, ETC. 



Fig. 15*. Transverse section through the genital plate, magnified. 



16. The central part of the aboral surface of an arm-bone of Ophiothrix 

 variabilis not far from the disk, magnified. The apophysis has a tri- 

 angular depression at its root for the umbo of the next bone ; below is 

 the median ridge ending in a broad double-faced peg. On either side 

 of the ridge is a slender muscle, m, and at the side of the muscle 

 remote from the median line the indefinite swelling, k. 



17. The same view of an aboral surface, further out in the arm ; the median 

 ridge is nodular and broad above ; the peg is double-faced and broad. 

 The position of the muscle is on the edge of the sockets on the side of 

 thf median ridge. Magnified. 



18. Tne adoral surface of the arm- bone opposed to fig. 17 : u, the umbo, 

 with a descending short part and a wide upper expansion ; k, the 

 knobs ; between them is the arched hollow for the double-faced broad 

 peg of the opposed bone. Magnified. 



19. An adoral surface nearer the disk, and corresponding nearly with 

 fig. 16 : s, the upper part of the apophysis, with a slot or groove for 

 the reception of the aborally placed apophysis ; u, the large broad umbo, 

 with a hollow below, and the knobs are placed on the side of the hollow 

 for the peg. Magnified. 



20. A view of the same from above obliquely downwards, showing the pro- 

 jecting umbo and knobs. The hollow below the umbo is not seen on 

 account of the foreshortening. Magnified. 



Plate XI. 



Fig. 21. Ophiothrix variabilis. The double-faced peg, magnified. 



22. The peg projecting, side view, magnified. 



23. Arm-bone of Ophiocampsis pellicula, aboral surface near the disk : 

 u d, the depression along the median line for the long umbo of 

 the adoral surface of the next bone. The knobs are large, and the 

 transverse ridges between the muscle-fiolds are well developed. 

 Magnified. Diagrammatic. 



24. An adoral surface : u, the long umbo, with a projecting convex part 

 above the lower notch ; depressions for the knobs of the opposed 

 surface on each side of the median umbo, just above the transverse 

 inter-muscle-field ridges. Magnified. Diagrammatic. 



25. Part of the aboral surface of an arm-bone of Ophiothrix variegatus, 



magnified, showing muscles attached to the sides of the apophysis, 



and passing transversely so as to be attached to the inner edge of 



the upper muscle-field. Magnified. 

 20. Muscle-fibres attached to the inner edge of the upper notch upon the 



upper muscle-field, magnified. 

 27. The apophysis, seen from above obliquely, showing the two layers of 



muscular fibres on the sides of its convex surface. Magnified. 

 (For description of figures 28-40 see page 100.) 



