REPORT ON THE CRINOIDEA. 01 



otliers rather resemble Rhizocrinus and Bathycnnus in the arrangement of the digestive 

 tube. Pentacrinus decorus is one of the hitter. Even at the level of the radial axillaries 

 the gut appears in section as a simple, but spacious cavity, with slight extensions at two 

 points round the connective, or rather calcareous, tissue in which the plexifonn gland is 

 embedded. But it could hardly be called kidney-shaped, as it is in Rhizocnnus and 

 Bathycriniis. 



Some sections of a Pentacrinus disk, that were made for Sir Wyvillc Thomson by 

 Dr. Stirling, show the indentation of one wall of the gut at the level of the radial 

 axillaries by the plexiform gland and its surroundings to be considerably more marked 

 than in Pentacrinus decorus (PL LVIII. fig. 4).^ I am unfortunately unable to determine 

 the species, as the sections were not properly labelled, and the series is not sufiiciently 

 complete for the purpose. 



In Pentacnnns wyville-thomsoni, however, the condition of the gut is much more like 

 that found in Antedon, which it further resembles in its disk being somewhat more inde- 

 pendent of the skeleton than in other Pentacrinidae. Even at the level of the second 

 radials, a horizontal section of the disk shows a strong concavity at one side of the gut, 

 which is thus almost crescentic in outline. The plications of the inner wall, however, 

 are by no means so well developed as they are in many Comatulse. 



In the remarkable genus Actinometra, the radial centre of the water-vascular system 

 does not correspond with that of the dorsal skeleton ; and the curious duality of the 

 Crinoid organisation is thus seen more distinctly in this type than in any other. The 

 mouth is not sub-central but excentric, or even marginal (PI. LXI. fig. 2), and there is 

 no regular symmetry in the distribution of the amliulacra (PL LV. figs. 1,2; PL LVI. 

 fig. 7). The mouth may be radial, as in aU endocycHc Crinoids, and such species of 

 Actinometra as Actinometra Solaris, Actinometra pidchella, and Actinometra jukesi 

 (PL LV. figs. 1, 2); or it may be interradial, as in Actinometra magnijica (PL LYI. 

 fig. 7) ; while in some types its exact position is difficult to determine. This is, 

 however, immaterial as regards the course of the digestive tube, which proceeds directly 

 downwards to a point somewhat behind and on the left {e(tst) of the centre of the disk, 

 and then commences to wind. 



Its direction, just as in the endocyclic Crinoids, follows the watch-hand when seen 

 from the ventral side ; but there are four coils instead of one. This is shown in fig. 3, 

 where the -f at the end of the first coil marks the termination of that part of the gut 

 which represents the whole digestive tube in the endocyclic forms. 



This first coil occupies the extreme edge of the lowest part of the disk, and consequently 

 passes in front of the mouth, so as to appear beneath it in longitudinal section (PL LX]. 

 fig. 2). The second coil passes immediately behind it, and is followed by two more in an 

 ever narrowing Ijut ascending spiral, which terminates in the more or less central anal 



' This fi,:;ure nearly corresponds to the southea?t comer of PI. \\\\\ tig. 7. 



