290 THE VOYAGE OF H.M.S. CHALLENGER. 
the relatively great proportion of length to width, whatever may be the shape of the full- 
grown joint. In some Comatule this condition is permanently retained, but in others 
and in the Pentacrinidz the arm-joints of mature individuals are usually much wider than 
long. In accordance with this, we find that even in young individuals of Pentacrinus 
decorus with quite high radials and arms only 12 mm. long, consisting of about a 
dozen properly formed joints, the great relative width of the latter is already very 
distinctly indicated (Pl. XXXYV.); while the last few joints are much smaller than their 
predecessors, and have rather the appearance of a pinnule than of the continuation of the 
arm, the preceding joint of which looks like an axillary bearing two small arm-stumps. 
This mode of development is less marked in a somewhat older individual of 
Pentacrinus decorus with about forty arm-joints, the later ones of which gradually 
decrease in size instead of becoming abruptly smaller; and the imperfect state of 
development of the later pinnules which is so characteristic of the Pentacrinide is 
very well shown. This would seem to indicate that the mode of growth of the arm-bases 
proceeds on a different plan from that of their middle and outer portions. In still larger 
individuals, as in the youngest specimens of Pentacrinus wyville-thomsoni (Pl. XVIIL. 
fig. 3), Pentacrinus naresianus’ (Pl. XXXa. fig. 1), and Metaerinus nodosus (Pl. LI. 
fig. 1), the terminal arm-joints are distinctly longer than wide, although the lower ones 
have almost assumed their adult form; and in all of them, but especially in Metacrinus 
nodosus, there is the characteristic reduction in the size of the later pinnules. Nothing is 
to be learnt regarding the order of the pinnule-succession in the Pentacrinidz from any of 
these young individuals; for the smallest of them are larger than some young Comatulee 
already detached from the stem, but without pinnules on the arm-bases, and they all have 
their proper complement of pinnules. The stem-joints of the immature Pentacrinide, 
like the later joints of growing arms, are relatively high in proportion to their width. 
This is exactly the reverse condition to that of the young joints formed immediately 
beneath the calyx (Pl. XXXIV. fig. 9). The same distinction appears in the very different 
type of stem characteristic of the Comatula-larva and of the Bourgueticrinide (Pl. VII. 
fig. 11; Pl. VIIa. fig. 1; Pl LIII. figs. 7, 8). The young joints are at first discoidal, 
then lengthen out, and finally the width increases relatively to the length so as 
sometimes even to exceed it considerably. The two types of stem are so very different 
that it is perhaps a little rash to reason about the one on the basis of the other. The 
interealation of new joints, which is so characteristic of the Pentacrinide, seems never to 
occur in the Bourgueticrinide, new joints being only formed beneath the calyx. In this 
last respect, however, the mode of growth in the youngest Pentacrinide with very 
slender stems appears to be very much what it is in the equally slender Rhizocrinus and 
Bathycrinus. But as the diameter of the stem increases to 3 or 4 millimetres the 
1 The arm-joints of this species are more like those of the Comatule than is the case in any other Pentacrinus. 
Instead of being nearly oblong, they have somewhat oblique ends, especially in the lower parts of the arms 
(Pls. XX VITI.-XXX.). 
