ANATOMY OF OPHIOTHRIX VARIABILIS, ETC. 



117 



forms. Ludwig appears to be the only describer of retractores, 

 of which he gives a very diagrammatic view. 



The little slips of fibres, possibly muscular, which connect the 

 genital scale with the genital plate and the radial shield are 

 formed of very delicate fibres which differ materially from those 

 of the adductores for instance, but neither set is striated. It will 

 be interesting to seek these unimportant slips in other species 

 of Opliiothrix. (I have not found them in the British species.) 



The muscles of the upper and lower fields of the arm-bones 

 are large, and are readily seen in broken-across joints. There is 

 a good deal of connective tissue, more or less fibrous, extending 

 from the adoral to the aboral surfaces of opposed bones ; and it 

 is attached at the edges of the median projection and of the 

 lateral continuation of the aboral surface, and to the edges of 

 the umbo and median edges of the muscle-fields in the adoral 

 surface. 



Besides this tissue two sets of muscles are visible in well- 

 preserved specimens. One set is a single stout fibre on either 

 side of the ridge leading down to the peg, and it arises above and 

 a little external to the socket for the knob of the adoral bone 

 (PI. X. fig. 16, m). Its direction is almost from within outwards, 

 and it is attached to the side of the depression below the umbo. 



The other set of muscles arise from the sides of the convex 

 aboral surface of the apophysis, and they pass on either side to be 

 attached to the edges of the upper muscle-field on the side of 

 the adoral surface. The fibres are numerous, and there appear 

 to be two sets of them, one being higher up than the other 

 (PI. XI. figs. 25, 26, 27). They appear to be necessary in order 

 to restrain the lateral dislocation of the apophysis. They occur 

 in all the Mergui Ophiothrices. 



VI. The Arm-bones of Ophiocampsis pellicula, Dune. 



The absence of upper arm-plates and the ability of the arms 

 to curl downwards would imply that the arm-bones could not be 

 made exactly upon the Ophiothrician type. The opposed surfaces 

 of the arm-bones are remarkable ; and the first thing that strikes 

 the eye is the enormous upper muscle-area on the aboral surface 

 of the arm-bones (PL XL fig. 23). The lower area is decidedly 

 small, and the tentacle-opening is large near the disk and smaller 

 further out. The next point to be observed is the upper and 

 aborally projecting apophysis, which is like that of Opliiothrix, 



I/Ore. JOUEy. — ZOOLOGY, VOL. XXI. 10 



