68 THE VOYAGE OF H.M.S. CHALLENGER. 
Pl. LVI. fig. 5), or it may be entirely bare to the naked eye, although microscopic 
examination usually reveals the presence. of calcareous spicules at the sides of the 
ambulacra. Its thickness and consistency vary greatly, more particularly in the arms. 
In Thaumatocrinus (Pl. LVI. figs. 2, 4), Atelecrinus, Promachocrinus, some species of 
Eudiocrinus, in many of the ten-armed Antedons of the temperate and Arctic Seas, and 
also in those from the greatest depths (1000 to 2900 fathoms), the ventral perisome of the 
arms is quite thin and delicate, except in the median line, where it is occupied by the 
ambulacra. These are consequently brought close down upon and between the muscular 
bundles, which are distinctly visible through the thin layer of perisome that covers them. 
In Actinometra, however (Pl. LV. figs. 1, 2; Pl. LVI. fig. 7), and also in the larger 
tropical Antedons, the ventral perisome of the arms is relatively thick and substantial, 
and no muscular bundles show through it at the sides of the ambulacra. These pass over 
the arm-bases on to the disk, where they are usually raised, sometimes considerably so, 
above the general level of its ventral surface. They converge towards the radial centre, 
where they unite into a smooth flattened space, the peristome ; and somewhere in this 
space, though not necessarily in its centre, is the opening of the mouth. 
In all recent Crinoids, with the single exception of the large Comatulid genus 
Actinometra,! the peristome is situated at or near the centre of the disk (Pl. III. fig. 2; 
Pl Vileto. 4) Plo Vil fie. 3; Pl X VIL sies, 6, 105 PL XXVI fies. 12; El eye 
fio 2) PL XXXIX. fig. 2; PL XLT fp 3; Pl BV.; Pl OGVE diga, 5.916): 
The same was doubtless the case, both in the fossil species of these genera, and also in 
the allied but extinct genera of Neocrinoids, e.g., Cotylecrinus, Eugeniacrinus, Apio- 
crinus, Marsupites, &c.; while the Blastoids and most, if not all, of the Paleocrinoids 
present the same character. 
In all the Comatule which have a central mouth, five primary ambulacra diverge 
from the peristome and fork more or less frequently according to the number of arms 
which they have to supply (PI. LV. figs. 3-7). This may also be the case in the Penta- 
crinide, more especially in those species which have a small number of arms (Pl. XVII. 
fic. 6; Pl, XXX. fig. 2; Pl. XXXII. fig 7; Pl XXXIV. fig. 25 EI ne 2). Bur 
in the multiradiate species the ambulacra of the disk are often quite irregular in their 
arrangement (Pl. XVII. fig. 10; Pl. XXVI. figs. 1, 2); and in the case of Metacrinus 
the ambulacra of the large lower pinnules sometimes start directly from the peristome 
or from one of the large primary groove-trunks, instead of from one of the subdivisions 
of the latter (Pl. XX XIX. fig. 2; Pl. XLIIIL. fig. 3; Pl. L. fig. 2). 
The brachial ambulacra of the regular Crinoids, which have no very great develop- 
ment of limestone in their ventral perisome, are usually well defined and bounded 
laterally by elevated folds of the perisome. The edge of each fold is cut out into a 
series of minute marginal leaflets ; while at the base of each of these, and to some extent 
1 See Appendix, Note C. 
To Ae ee Be 
