EEPORT ON THE CTJNOIDEA. 03 



(PI. XVI. figs. 2, 3 ; PI. XVII. fig. 1), with its extraordinarily narrow arm-groove, the 

 ambulacra! plating of the pinnule always arises from a similar hut less defined skeleton 

 on the brachial ambulacrum (PI. XVII. figs. 7-9 ; PI. XXXIII. figs. 3, 4 ; PL XLI. 

 figs. 4, 13; PI. 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 (PI. XXVII. 

 figs. 6, 13 ; PI. XLI. figs. 4, 13), are in like manner connected witli the anambulacral 

 system over the arm-bases (PI. XXVI. figs. 1,2; PI. 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 

 Actinocrinns ; 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 tt) me to have been much nearer the truth when he suggested 

 that the many little branches of the bifurcating arms in Cyathocrimts 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, weU aerated water. He alludes to the importance 

 of the pinnated arms in bearing a vast aggregate of tubular tentacles by which 

 respiration is eflected ; 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 efi"ected 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 is no reason why the 

 genital glands should not have l)een contained in these pinnule-less arms, for they 

 frequently extend from the pinnules down into the arms both in Holoims (PI. Vc. 

 figs. 1, 2, ov.), in many Comatula3 (PI. LXI. fig. 3), and even in Pentacrinidaj ; 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 



^ Notes on the Internal and External Structure of Palssozoic Crinoids, ^mcr. Journ. Sci. and Arts, vol. xiv. p. 120. 

 2 Phil. Trans., 1866, pp. 701, 702. 



