90 THE VOYAGE OF H.M.S. CHALLENGER. 
The further course of the rectum is shown in Pl. VIIb. fig. 8, which represents a 
section through the upper part of the disk at the level of the articulation between the 
first and second brachials ; and the last trace of the plexiform gland (z) is seen between 
the fore-gut and the final coil of the intestine. 
Both Rhizocrinus and Bathycrinus exhibit a peculiarity in the relations of the 
digestive tube and visceral mass which does not appear in the Pentacrinoid. The large 
crests at the sides of the arm-groove in the second brachials of Rhizocrinus, which were 
well figured by Sars," have always been a puzzle to me, for they are something more 
than unusual developments of the muscle-plates at the distal end of the joint. Horizontal 
sections of the cup show, however, that they support the sides of comparatively large 
interradial diverticula from the intestinal coil, the outer ends of which are overlapped to 
some extent by the broad dorsal surfaces of the brachials (Pl. VIIIa. fig. 8). 
The first traces of these extensions of the digestive cavity appear in Rhizocrinus lofo- 
tensis at the level of the muscle-plates of the first brachials, by which they are in a great 
measure supported. They become more independent of the skeleton at the syzygy with 
the second brachials ; but enter into close relations with these joints at their distal ends, 
where the crests on the ventral surface become more prominent. They are much more 
marked in some individuals than in others, but the outline of the visceral mass is 
always distinctly pentagonal, even if its angles be not produced outwards, so as to give 
it a stellate appearance. The same peculiarity appears in Bathycrinus. The crests on 
the radial axillaries are large, wing-like processes, altogether distinct from the muscle- 
plates to which the first brachials are attached, as is shown in PI. VII. fig. 4a, and Pl. VIIa. 
fig. 17; and they afford a large amount of support to the interradial diverticula of the gut 
(Pl. VIIb. fig. 7). These develop themselves gradually from below upwards, no trace 
of them appearing below the articulation of the second and the axillary radials; but 
indications of them are still visible at the upper part of the first brachials, as shown in 
Pl. VIIb. fig. 8. 
In this figure, too, may be seen the earlier and simpler stages of that plication of the 
inner or adcentral wall of the gut which is so much more marked in Pentacrinus, and 
still more so in the Comatule, especially in Antedon eschrichti, Promachocrinus, and 
allied forms (Pl. LVIII. figs. 4-6). 
The disk of the Comatulz is generally flatter than that of the stalked Crinoids, and is 
almost entirely independent of any lateral support from the second radials. These lie 
beneath more or less of its dorsal surface, but do not protect its sides. 
The descending portion of the fore-gut is therefore comparatively short, and the 
coiled intestine into which it passes lies spread out on the upper surface of the radials; so 
that there is no general dilatation of the gut at the bottom of the cup as in Rhizocrinus, 
Bathycrinus, and the Pentacrinoid. Some of the Pentacrinidee approach Antedon, while 
1 Op. cit., figs. 55-57. 
