REPORT ON THE CRINOIDEA. 209 
Shortly before his death Sir Wyville Thomson placed in my hands a portion of the 
ray represented in Pl. Vb., with the request that I would cut it into sections for him. I 
found this to be an exceedingly difficult task, partly because of the rolled-up condition 
of the arms, and partly because the calcareous substance of the skeleton is so much 
denser than that of other Crinoids; so that the organic basis which is interpenetrated by it 
and remains behind after decalcification, has nothing like the consistency that we meet 
with in the corresponding parts of the Comatule or of Bathycrinus. The presence of 
large bundles of muscles and ligaments without any helping syzygies also increases the 
difficulty of all attempts to obtain thin sections. But although I was not so successful 
as I could have wished, I was able to determine satisfactorily that the anatomy of a 
Holopus-arm is similar in all essential respects to that of an ordinary Crinoid (Pl. Vb. 
fig. 1; Pl. Ve. figs. 1, 2). The axial cord traversing the central canal of the skeleton 
gives off its pinnule branches in the usual way, #.e., alternately on opposite sides. These 
branches have a long distance to go before they reach the pinnules, owing to the 
attachment of the latter on the upper edges of the large muscle-plates. As long as the 
branch remains in the substance of the arm-joint it does not take a straight course as is 
the case in the other Crinoids, but is thrown into a series of loops in a dorsoventral 
direction (Pl. Ve. fig. 2, a), and after it enters the pinnule its course is still somewhat 
sinuous (Pl. Ve. fig. 3, @). 
These branches, like the main arm-trunk, are relatively of very small size, which is 
perhaps to be accounted for by the fixed position of the animal. No swimming 
movements are of course possible, but only those of flexion and extension are performed 
by the arms. All the ambulacral structures of the Holopus-arm are lodged in the deep 
median groove of its skeleton, and are usually small in comparison with the great 
transverse diameter of the joints. The cceliac canal is situated, as usual, between the two 
large muscular bundles, with a small genital canal separating it from the single 
subtentacular canal above (PI. Vb. fig. 1). 
The epithelial lining is very much the same in character in all these canals, consisting 
of low flattened cells. According to Ludwig? this is also the case in Antedon eschrichti, 
but this statement is not borne out by his figures. In one figure” he represents a well 
marked cellular lining to the cceliac canal and subtentacular canal, but leaves the genital 
canal without any; though in a more magnified representation ® the wall of the genital 
canal bears an excessively delicate layer of much flattened cells, which consist of little 
more than nuclei. This is more in accordance with my own observations, for | have 
always found that the epithelial cells in the genital canal are much flatter and less easy 
to see than those in the cceliac and subtentacular canals. In Holopus, however, the 
difference is much less marked. The genital cord is of essentially the same nature as in 
other Crinoids; though it is of a much less branching character in the axillary radial 
1 Crinoiden, loc. cif., p. 29. 2 Thid., pl. xii. fig. 8. 3 Ibid., pl. xiii. fig. 13. 
(ZOOL. CHALL, EXP.—PART XXx1I,—1884.) Li 27 
