364 THE VOYAGE OF H.M.S. CHALLENGER. 
exceedingly regular, as it almost invariably has six arms above each distichal axillary 
arranged in the following order 2,1; 1, 2. There are only two instances of an axillary 
occurring on one of the inner pair of the four tertiary arms; while it is never absent on 
one of the outer pair. This regular arrangement recalls that already noticed in some 
species of Pentacrinus. But it is much less constant than in the more normal example 
with the regular calyx, and it is in this individual that the solitary instance of a fifth 
axillary occurs; while the peculiar development of spines on the lower arm-divisions is 
much more marked in the one without a base. 
The pinnule-ambulacra vary considerably in their appearance according to their posi- 
tion, as will be seen by a comparison of Pl. XLVII. fig. 13 and Pl. XLIX. figs. 6, 7; and 
it is only on the later pimnules of the arm that the side and covering plates come to be 
distinctly differentiated. The peculiar mode of development of the side plates on some 
of the larger pinnules has been already described (ante, p. 82). The same character is also 
visible in Metacrinus wyvillii and Metacrinus nodosus (PI. LI. fig. 12), though to a less 
extent. 
The occasional enlargement of some of the arm-joints appears to indicate that an 
encysting Myzostoma had commenced operations. This is probably the Myzostoma 
pentacrint of von Graff! It “ does not produce real cysts upon the arms of its host, but 
only swellings of several (three to six) joints, which gradually disappear.” It was found 
in abundance on Pentacrinus alternicirrus from the same Station (214) as Metacrinus 
costatus ; whereas the free Myzostoma wyville-thomsoni which was attached to the disk of 
the latter species occupied a similar position on the disk of Metacrinus angulatus from 
Station 192, off the Ki Islands. 
9. Metacrinus nodosus, n. sp. (Pls. L., LI.). 
Dimensions. 
Length of stem to seventeenth node, : : é : . 14:00 cm. 
Diameter of stem, . ‘ : F : 5 ‘ 3°00 mm. 
Longest cirrus (forty-five acne : ; : : . 5 oho 53 
Diameter of calyx, . : : ; ; $ : : 6:00 
Diameter of disk, . * ‘ . : =) 12sD0 Fe; 
Length of pinnule on fourth radial fourteen’ joints), | 3000. 
Length of pinnule on first joint after palmar axillary Gime hue) : 9:00) 5 
Stem slender and sharply pentagonal. Light or nine internodal joints with distinctly 
crenulated edges and a faint tubercle in the middle of each side. Nodals, especially in 
the young stem, produced outwards at the angles between the wide cirrus-facets which 
do not reach either edge. Supra- and infra-nodal joints both somewhat modified.  Cirri 
1 Zool. Chall. Exp., part xxvii. p. 68, 1884. 
