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



one or two of them whicli may perhaps admit of this interpretation. But not even this 

 can be said of the originals of figs. 3 and 4 on PL III. I think, however, that the evidence 

 detailed above is sufficient to bear out the statement that Holojnis has three radials, of 

 which the two outer ones are united by syzygy. We should accordingly expect to find 

 a similar syzygial union between the first and second brachials ; bi;t of this there is no 

 evidence whatever. The distal face of the first and the proximal face of the second 

 brachial (PI. III. figs. 3, 7) present the ordinary characters of a muscular joint. There 

 are, indeed, in the small specimen shown in PL IV. some traces of lines crossing the first 

 brachials, which might be taken as indicating a syzygial union of two primitive joints ; 

 but they are nothing like as distinct as those in the radials. I think, therefore, that for 

 the present, at any rate, we must regard Holopus as an exception to the general rule 

 which holds good in other Crinoids, as to the similarity between the modes of union of the 

 two outer radials and the two lower brachials respectively {ante, p. 49). It is further 

 remarkable from the fact that there seem to be no syzygies between any of the other 

 arm-joints. 



The outer surfaces of the composite radial axillaries were described by Sir Wyville 

 Thomson^ as "very gibbous, thrown out into almost hemispherical projections, studded 

 with low tubercles" (PL III. figs. 3-5). They are produced dorsally a considerable 

 distance beyond the edges of the articular faces, as is the case with all the lower arm- 

 joints (PL III. figs. 6-13); and they fit very closely against their fellows, their sides 

 being flattened and more or less marked by ridges and furrows, which interlock with 

 those on the adjacent axillaries. These furrows are also apparent on the sides of the 

 lower arm-joints (PL II. ; PL III. figs. 6-12 ; PL Va. fig. 3 ; PL Vb. fig. 4). The 

 muscle-plates of the axillaries, and in a less degree also those of the arm-joints, are greatly 

 thickened, and their upper edges are cut out into coarse teeth. This is well shown in the 

 right-hand figure on PL II. and in the upper part of PL III. fig. 2, where some of the 

 adjacent axillaries are seen interlocking wdth each other. 



In all the specimens of Holopus yet known, including the fresh fragment dredged by 

 the " Blake " ofi" Montserrat, the arms are strongly recurved, and by their close mutual 

 apposition conceal the disk entii-ely (Pis. I., II., IV.; PL Va. fig. 3 ; PL Vb. figs. 4, 5). 

 Obviously, however, this cannot be the natural condition of the living animal. There is a 

 large food-groove on the upper surface of each arm and pinnule (PL Va. fig. 1 ; PL Vb. 

 figs. 1, 4, 5) ; and there is every reason to think that the living animal, when undisturbed, 

 spreads out its arms with the ventral surface upwards just as other Crinoids do for the 

 purpose of obtaining food. The large size of the paired flexor muscles uniting the joints 

 (PL VI3. fig. 1, m) would seem to give the power of rolling in the arms very rapidly 

 and completely, so as to aftbrd the utmost protection to the soft parts contained 

 within the cup ; while the small, but very close and compact bundles of elastic 



1 Loc. cit., p. 408. 



