62 THE VOYAGE OP H.M.S. CHALLENGER. 



just as on the feathery arms of Antedon eschrichti and hosts of other ComatulaB ; and 

 they are obviously of the same nature as the pinnules of Neocrinoids generally. " When 

 the arms are closed, the two series of pinnulfe 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 pinnulse had 

 the same functions, partly at least, as the alternate plates in Cijathocriniis, &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 pinnulse."^ 



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 ; PI. XIV.; PL XVII. fig. 1 ; PL XXVII. 

 fig. 13; PL XXXIII. fig. 3; PL XLI. fig. 11; PL XLVII. fig. 12; PL LIV. fig. 1). 

 As regards the last paragraph, I 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 Pentacrinidse and Comatulse (PL Vc. figs. 9, 10; 

 PL Villa, fig. b—cp. PL XIII. 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 LII. figs. 5, 6 ; PL LIV. figs. 4, 6-9). Similar plates 

 occur on the pinnules of Actinocrinidaj and Platycrinidse, their grooves being " covered 

 by a double series of very minute pieces, though, owdng 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 Vc. figs. 8-10 ; 

 PL VI. figs. 1-4 ; PL VII. figs. 2, 7, 8 ; PL VIII. figs. 3, 5 ; PL Villa, fig. 1 ; PL IX. 

 fio's. 1-4 ; PL X. fig. 20). The first of these has side plates on the pinnules (PL Vc. 

 figs. 9, 10, sp.), but they are not distinctly differentiated on the arms; and the same is 

 the case in the Pentacrinidse and Comatulaj. But except in Pentacrinus maclearanus 



' Revioion, part i. p. 25. - nid., part ii. p. 25. = Ibid., p. 24. 



