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 Comatulse this condition is permanently retained, but in others 

 and in the Pentacrinidse the arm-joints of mature individuals are usually much wider than 

 long. In accordance with this, we find that even in young individuals of Pentacrimis 

 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 (PI. XXXV.) ; while the Jast few joints are much smaller than then- 

 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 al^out 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 Pentacriniclje 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 Pentacrimis iri/ville-thomsoni' (PI. XVIII. 

 fig. 3), Pentacrinus naresiauus^ (PL XXXa. fig. 1), and Metacrinus nodosus (PI. 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 Pentacrinidae from any of 

 these young individuals ; for the smallest of them are larger than some young Comatulse 

 already detached from the stem, but without pinnules on the ai'm-bases, and they all have 

 their proper complement of pinnules. The stem-joints of the immature Pentacrinidse, 

 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 Conuttula-larva, and of the Bourgueticrinidse (PL VII. 

 fig. 11 ; PL Villa, 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 

 intercalation of new joints, which is so characteristic of the Pentacrinidse, seems never to 

 occur in the Bourgueticrinidse, new joints being only formed beneath the calyx. In this 

 last respect, however, the mode of growth in the youngest Pentacrinidse with very 

 slender stems appears to be very much what it is in the equally slender Rhizocrinus and 

 Bathycrinu-s. But as the diameter of the stem increases to 3 or 4 millimetres the 



• The arm-joints of this species are more like those of the Comatulae 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 

 (Pis. XXVIII.-XXX.). 



