366 THE VOYAGE OF H.M.S. CHALLENGER. 
modification of the lower arm-joints above the pinnule-sockets, which was noticed 
in Metacrinus costatus, is also visible in Metacrinus nodosus, though to a less 
extent. 
There is a good deal of difference, however, between the stems of the two species. 
While the normal number of internodal joints in Metacrinus costatus is seven or eight, 
that of Metacrinus nodosus is eight or nine, and they are very regularly marked by a 
faint tubercle in the middle of each side, which is flat and scarcely hollowed at all (Pl. LI. 
fig. 8). In Metacrinus costatus, on the other hand, the sides of this stem are almost 
smooth, or only marked by a few occasional horizontal ridges, while they are distinctly 
hollowed between the prominent angles (Pl. XLIX. figs. 3, 4), so that the stem appears 
to be traversed along its whole length by five rather sharp interradial ridges. In the 
uppermost and growing part of the stem these ridges are much more prominent on the 
closely set nodal joints than on the thin internodals which separate them. But as the 
latter increase in thickness their interradial angles are also enlarged, so that those on the 
nodal joints are not specially prominent. This character is also visible in Metacrinus 
nodosus, both in the stem-joints of young individuals (Pl. LI. figs. 6,7) and in the 
growing part of the stem of the more mature specimen. But there are no strong ridges 
developed at the angles of the internodes, as is the case in Metacrinus costatus, so that 
those of the nodal joints are always more or less prominent (PI. L. fig. 4; Pl. LI. fig. 8). 
The side plates on the pinnule-ambulacra of Metacrinus nodosus are relatively large 
and pointed (Pl. LI. figs. 11, 12). They are developed in the same style as those of 
Metacrinus costatus (Pl. XLVII. fig. 13) from the somewhat irregular plates of the 
brachial ambulacra, which are not so bifid as in Metacrinus angulatus (Pl. XXXIX. 
fig. 13) or in Metacrinus varians (P). XLVII. figs. 11, 12). 
The young individual of Metacrinus nodosus, which is represented in Pl. LI. fig. 1, 
has a shghtly tapering stem containing sixteen nodes, at the eighth of which one cirrus- 
socket is undeveloped. The characters of the young stem-joints, which are shown in 
figs. 2-7, have been noticed already (ante, p. 291). 
Of the four rays remaining in this specimen only one is normal, 7.e., composed of six 
joints, of which the second and fourth are syzygies. In one case the fifth joint is the 
axillary, and in another the fourth, which is at the same time a syzygial joint like the 
second ; while in another ray the fourth radial is not a syzygy, though the fifth is 
axillary and united to it somewhat closely, so as to give almost the appearance of a 
syzygy-' But the presence of a pinnule on the fourth joint shows conclusively that it 
cannot be the hypozygal of a syzygy, as its homologue is in the next ray. 
This specimen is so young that the palmar axillaries are with difficulty distinguished 
from the ordinary joints of the secondary arms; and in some cases at any rate they 
seem to have been farther from the distichal axillaries than is usual in the larger indi- 
1 A little too much has been made of this resemblance to a syzygy in the right hand side of the figure, 
