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REPORT ON THE CRINOIDEA. 65 
(Pl. XVI. figs. 2,3; Pl. XVII. fig. 1), with its extraordinarily narrow arm-groove, the 
ambulacral plating of the pinnule always arises from a similar but less defined skeleton 
on the brachial ambulacrum (Pl. XVII. figs. 7-9; Pl. XXXIII. figs. 3, 4; Pl XLI. 
figs. 4,13; Pl. LIV. fig. 7). This is itself directly continuous with the ambulacral 
plates of the disk; while the perisomic plates, which may appear at its sides (Pl. XX VII. 
figs. 6, 13; Pl. XLI. figs. 4, 13), are in like manner connected with the anambulacral 
system over the arm-bases (Pl. XXVI. figs. 1,2; Pl. L. fig. 2). Wachsmuth’s own 
beautiful observations have demonstrated the existence both of anambulacral and of 
- ambulacral covering plates on the upper surface of the body, beneath the vault of 
Actinocrinus; and since the latter also appear on the pinnules, it seems unreasonable 
to doubt their presence on the arms. 
But if, as I firmly believe, brachial covering plates occurred in Actinocrinus as in 
Cyathocrinus, what becomes of the supposed homology between these covering plates in 
the latter genus and the pinnules of the former type ? 
Mr. Wachsmuth appears to me to have been much nearer the truth when he suggested 
that the many little branches of the bifurcating arms in Cyathocrinus performed the 
functions of pinnules,’ though he gave no explanation as to what these functions were. 
In recent Crinoids, and most probably therefore in the fossil ones also, the functions of the 
pinnules are threefold, viz., (1) the protection of the fertile portions of the genital glands, 
which are all connected together by the sterile rachis in the arm; (2) respiration ; 
(3) alimentation. 
Dr. Carpenter’ has pointed out that the Crinoids are very closely dependent for the 
maintenance of their life upon pure, well aerated water. He alludes to the importance 
of the pinnated arms in bearing a vast aggregate of tubular tentacles by which 
respiration is effected; and regards it as probable “that the ordinary pinnules are 
specially related to the function of respiration, in virtue alike of their proper branchial 
canals, and of the ambulacral canals and the tubular tentacula with which they are 
furnished.” 
This process of respiration was doubtless effected just as well by the tentacles 
connected with the water-vessels in the many-branched arms of Cyathocrinus, as by 
those on the pinnules of Actinocrinus or Comatula; and there i8 no reason why the 
genital glands should not have been contained in these pinnule-less arms, for they 
frequently extend from the pinnules down into the arms both in Holopus (Pl. Ve. 
figs. 1, 2, ov.), in many Comatule (Pl. LXI. fig. 3), and even in Pentacrinide ; so that 
they often appear in section as taking the place of the sterile genital cord, which unites 
the more fertile portions of the gland that are contained in the successive pinnules. 
The third great function of the pinnules of a Crinoid arm is to present as large an 
1 Notes on the Internal and External Structure of Paleozoic Crinoids, Amer. Journ. Sci. and Arts, vol. xiv. p. 120. 
2 Plul. Trans., 1866, pp. 701, 702. 
