REPORT ON THE CRINOIDEA. 209 



Shortly before his death Sir Wyville Thomson phiced in my hands a portion of the 

 ray represented in PI. Vb., with the request that I would cut it into sections for him. I 

 found this to be an exceedingly difficult task, j)artly because of the roUed-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 Comatulse 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 

 Holojnis-aviai is similar in all essential respects to that of an ordinary Criuoid (PI. Vb, 

 fig. 1 ; PI. Vc. figs. 1, 2). The axial cord traversing the central canal of the skeleton 

 gives oiFits pinnule branches in the usual way, i.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 (PI. Vc. fig. 2, a), and after it enters the pinnule its course is still somewhat 

 sinuous (PI. Vc. fig. 3, a). 



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-Sirm. are lodged in the deep 

 median gi'oove of its skeleton, and are usually small in comparison with the great 

 transverse diameter of the joints. The coeliac 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 Ayitedon eschrichti, 

 but this statement is not borne out by his figures. In one figure ' he represents a well 

 marked cellular lining to the coeliac 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 I have 

 always found that the epithelial cells in the genital canal are much flatter and less easy 

 to see than those in the coeliac and subtentacular canals. In Holopus, however, the 

 difierence 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, he. cit., p. 29. - Ibid., pi. xii. fig. 8. ^ Ibid., pi. xiii. fig. 13. 



(ZOOL. CHALL. EXP. PAET XXXII. — 1884.) li 27 



