208 THE VOYAGE OF H.M.S. 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 IL, and also in PI. 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 

 (PI. Vc. 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. 3, 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 Vc. figs. 9, 10; PL XLIX. figs. 6, 7; PL LI. figs. 11, 12; PL LIV. 

 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 (PL Va. figs. 1, 2. PL Vb, 

 fig. 2 ; PL Vc. 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 diflicult 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 eschrichti, though thrown into much stronger corrugations at the outer ends 

 of the tentacles. 



