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PROF. P. M. DUNCAN ON THE 



muscle arises in each of the depressions (fig. 8, c), and the fibres 

 pass adorally through the foramen in the jaw-plate, which is in- 

 ternal to them, and are attached to one side of the hollo wed-out 

 base of a tooth (figs. 3, 6, 7). The muscles are radial in direction, 

 and there are two of them to each tooth, and eight in all. They 

 are evidently the interradiales adorales superiores ; and their 

 action, by becoming tense and contracting, would be to fix the 

 teeth tightly on the jaw-plate, so that they could project hori- 

 zontally. On the other hand, as the muscles become lax, the bases 

 of the teeth would be less tightly in contact with the jaw-plate, 

 and some movement, up and down, of the free edge of the teeth 

 would be possible. A muscle attaches the outside edges of the 

 jaw-plate, on the brachial as well as the interbrachial sides, to the 

 vertical processes (jaws) or adoral ends of the mouth-frames 

 in the lower half ; it arises from the oblique grooves near the 

 edge, and is inserted on the edge of the jaw-plate. There are, 

 according to Simroth, muscles between the vertical broad groove, 

 formed by the union of the adoral surfaces of the jaws (fig. 8), 

 and the aboral surface of the jaw-plate, and it is possible that 

 they exist in Ophiothrix variabilis, but I have not verified the 

 fact ; at the same time there is connective tissue in the vertical 

 groove uniting the conjoined skeletal parts. 



The " interradiales aborales " (PI. X. fig. 1, mi) are the largest 

 and most important muscles of the mouth-frame regions ; the fibres 

 are not radial in direction, but conform in direction to the circle 

 formed by the adjacent mouth-frames. The fibres arise on the 

 interradial flank of one, and are inserted into the flank of the 

 opposite mouth-frame, that is, on the other side of the inter- 

 brachial space. The greater part of the flanks is covered, and 

 the fibres extend below to between the unjoined edges of the 

 side mouth-shields, and touch the adoral edge of the mouth-shield 

 at the median line. 



I have not found any muscular fibres in the space between the 

 brachial flanks of the mouth-frames, such as the " radiales " of 

 Simroth, although their existence is evident in Ophiaclis. 



It is not necessary to do more than mention the existence of 

 the adductor and retractor muscles of the genital plate and 

 radial shield, since; they have already been described. They are 

 not mentioned by Simrotli as occurring in his type. Lyman 

 given Home excellent drawings of the adduetores, but does not 

 mention the retractor, which is, however, very visible in some 



