208 THE VOYAGE OF H.M.S8. CHALLENGER. 
and following brachials are much larger and have broad lower joints that gradually come 
to take up more and more of the whole surface of the arm-joints to which they are 
attached. In fact the bases of the pinnules of alternate joints that are borne upon the 
same side of the arm are only just separated from one another by the narrow ends of 
the intervening joints, which have their pinnules on the opposite side of the arm. This 
is well shown in the right-hand figure on Pl. I1., and also in Pl. Va. fig. 3. The pinnules 
are rolled in upon themselves (PI. III. fig. 16) exactly in the same way that the arms 
are (PI. Va. figs. 1, 2). The four or five lower joints are very broad, but the rest of the 
pinnule tapers away rather rapidly. The joints are united by paired muscular bundles 
(PL Ve. fig. 2, m), which is a somewhat unusual condition. 
The disk of Holopus is unfortunately still but very imperfectly known, and I have 
only been able to examine it in one specimen. The central mouth is protected by five 
large and triangular oral plates which are opposite to the clavicular pieces of the united 
radials (Pl. III. fig. 2). The lateral edges of each of these plates are thickened and some- . 
times more or less cut into false teeth ; while the raised central portion is pierced by from 
fifteen to twenty minute holes, the water-pores. The bases of the orals seem sometimes 
to rest directly against the edge of the radials; while they are sometimes separated from 
this edge by an irregular row of small triangular plates. It is not unlikely that an 
anal tube is concealed somewhere or other among these plates, as in the case of Hyocrinus 
(Pl. VI. figs. 8, 4); but I have seen no certain traces of it in the dry specimen. The 
same would probably be the case with Hyocrinus under similar conditions. 
The food-grooves which come away from the mouth between every two of the oral 
plates are continued out on to the axillaries and from thence on to the arms. They 
occupy the deep channel between the large muscular processes at the sides of the joints, 
and in the dry specimen appear to be bordered by small, irregular plates. These, how- 
ever, do not seem to correspond either to the side plates or to the covering plates of other 
Crinoids \(Pl. Vie. figs.9, 10: ePl) Que fies 76/76 PL a, digs’ ai, 6125 Play 
figs. 4, 6-9); for an examination of spirit specimens shows that these small plates really 
belong to the tentacles, which are relatively large and stout (PI. Va. figs. 1, 2. Pl. Vb. 
fig. 2; Pl. Ve. figs. 1-3—T). The bases of these tentacles are protected by scale-like 
plates formed of the usual calcareous reticulation (Pl. Vb. figs. 2, 3). They are not easily 
made out at the side of the arm-groove, but on the lower parts of the pinnules there seem 
to be from two to three tentacles on either side of each joint. It is difficult to get a correct 
estimate of their absolute size ; but after careful comparison with an eyepiece micrometer 
I should judge them to be nearly twice the size of the largest that I could find in any 
preparations of Antedon eschrichti. The general arrangement of the tentacles is the same 
as in other Crinoids; but the epithelial layer covering them is, if anything, thinner than 
in Antedon eschricht, though thrown into much stronger corrugations at the outer ends 
of the tentacles. 
Py 
