364 THE VOYAGE OF H.M.S. CHALLENGER. 



exceedingly regular, as it almost invariably has six arms above each clistichal axillary 

 arranged in the following order 2, 1 ; 1, 2. There are only two instances of an axillarj'- 

 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 uormal example 

 wth 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 PI. XLVII. fig. 13 and PI. XLIX. figs. 6, 7 ; and 

 it is only on the later pinnules of the arm that the side and covering plates come to be 

 distinctly difi'erentiated. 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 tvyvillii 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 

 pentacrini 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 ivyville-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. (Pis. L., LI,). 



Dimensions. 



Length of stem to seventeentli node, .... 



Diameter of stem, ....... 



Longest cirrus (forty-five joints), ..... 



Diameter of calyx, ....... 



Diameter of disk, ....... 



Length of pinnule on fourth radial (fourteen joints), 



Length of pinnule on first joint after pabnar axillary (thirteen joints), 



Stem slender and sharply pentagonal. Eight 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. 63, 1884. 



