62 THE VOYAGE OF H.M.8. CHALLENGER. 
= 
just as on the feathery arms of Antedon eschrichti and hosts of other Comatule ; and 
they are obviously of the same nature as the pinnules of Neocrinoids generally. “‘ When 
the arms are closed, the two series of pinnulz of one arm are laid upon each other so 
neatly, that the arm-furrow must have been thereby perfectly shut off from the surround- 
ing water. No additional covering has yet been observed in these genera, and it was 
evidently unnecessary. All this seems to point to the conclusion that the pinnule had 
the same functions, partly at least, as the alternate plates in Cyathocrinus, &c., and as 
both have the same position, and evidently could be opened and closed by the animal, 
we do not hesitate to consider the latter as the homologue of the former, or in fact 
as rudimentary pinnulz.”* 
The first sentence of the above passage concerning the pinnules of Actinocrinus and 
Platycrinus would apply equally well to any Comatula or Pentacrinus, whether the 
ambulacra be plated or not (Pl. XIII. fig. 13; Pl. XIV.; Pl. XVII fig. 1; Pl. XXVII. 
Hes 5 Pl IT, fie. 33) Pleo Mil feo dds Ply VL, fete Aa atte, a) 3 
As regards the last paragraph, 1 cannot help thinking that it affords an instance in 
which analogy has been mistaken for homology. 
The overlapping of the pinnules so as to cover in the ambulacra may occur in all 
recent Crinoids; while the grooves of the pinnules themselves, like those of the arms 
and disk, are often bordered by two more or less distinct rows of minute movable 
alternating plates, the “covering plates.” These may themselves be supported on 
“side plates,” thus making four rows in all, which are sometimes very fully developed 
as in Hyocrinus, together with many Pentacrinidee and Comatulee (Pl. Ve. figs. 9, 10; 
Pl. VIlla. fig. 5—cp. Pl. XIIL figs. 15,16; Pl. XVII. fig. 8; Pl. XXVII figs. 4-6, 11-13; 
Pl. XXXIII. figs. 1-4; Pl. XLI. figs. 4, 11-13; Pl. XLVII. figs. 10-12; Pl. XLIX. 
figs. 6, 7; Pl. LI. figs. 11,12; Pl. LIL figs. 5,6; Pl. LIV. figs. 4, 6-9). Similar plates 
occur on the pinnules of Actinocrinidee and Platycrinidee, their grooves being “ covered 
by a double series of very minute pieces, though, owing to defective preservation, this 
covering is rarely observed.” These pinnules “fit together so neatly and cover the 
arm-furrow so perfectly that additional plates were scarcely needed.”” 
A teleological argument of this kind is, however, no proof that the brachial ambulacra 
are unprovided with plates in Actinocrinus and Platycrinus, when there is a double 
series on the pinnules which they bear; and, as a matter of fact, the evidence afforded 
by the Neocrinoids is all against this view. Hyocrinus, Bathycrinus, and Rhizocrinus 
all have covering plates on the arms as well as on the pinnules (Pl. Ve. figs. 8-10; 
PL VE figs. 1-4; (Pl. VIL. figs. 2,.7, 85 PL VILL figs. 3."oeee ey iia. ies os PL alee 
figs. 1-4; Pl X. fig. 20). The first of these has side plates on the pimnules (PI. Ve. 
figs. 9, 10, sp.), but they are not distinctly differentiated on the arms; and the same is 
the case in the Pentacrinide and Comatule. But except in Pentacrinus maclearanus 
1 Revision, part 1. p. 25. 2 Tbid., part ii. p. 25. 3 Ibid., p. 24, 
