64 THE VOYAGE OF H.M.S. CHALLENGER. 



area as possible of the ciliated ambulacral grooves on tlieir ventral surface in order 

 to catch the minute organisms in the surrounding water which might serve as food, and 

 send them down the ambulacra of the arms towards the central mouth. For this purpose, 

 as for that of respiration, the repeated branching of the long arms of Ci/athocrinus would 

 be as eflfective as the development of pinnules on the successive joints of less divided arms 

 in other Crinoids. The three great functions of these pinnules would thus have been 

 performed without difficulty by the branching arms of Cjjathocriniis. Biit for which of 

 them are the covering plates of the arm -grooves at all adapted, and how far can these 

 plates be considered as repetitious of the arms on a small scale? To each of these 

 questions only a negative answer is possible. 



The covering plates of recent Crinoids may be found closed down over the food-groove 

 after death (PI. XIII. fig. 16 ; PL XVII. fig. 7 ; PI. XXVII. fig. 12 ; PI. XXXIX. 

 fig. 12 ; PI. XLVII. fig. 10 ; PL LI. fig. 12 ; PL LIL fig. 6; PL LIV. figs. 4, 6); but they 

 are just as often met with in a more or less erect position, thereby opening the food- 

 groove to the exterior (PL Vc. figs. 8-10; PL Villa, fig. 5— cp. PL XVII. figs. 2, 8, 9; 

 PL XLVII. figs. 4, 13; PL LI. fig. 11 ; PL LIV. figs. 7-9). Just in the same way the arms 

 are frequently closed round the disk in the dead animal (Pis. XVIII. , XIX., XXV., XXVIII. , 

 XXXI., XLV., XLIX., LIL) ; while in other cases they are more or less expanded, as 

 they were during life (Pis. XXXIV. , XL. . XLIL). Messrs. Wachsmuth and Springer argue, 

 however, (l) because the arm-groove of the fossil Cijathocrirms is closed by covering 

 plates which could be opened and closed by the animal (as it is in the dried arm-fragment 

 of Pentacrinus asterius which is shown in PL XVII. fig. 7), and (2) because the arm- 

 groove of Actinocrinus must have been perfectly shut ofi" from the surrounding water 

 by the apposition of the pinnules ivhen the arms loere closed; therefore the covering 

 plates of Cijathocrinus are homologous with the pinnules of Actinocrinus. 



But what advantage is it to the animal to have its arm-grooves closed up, whether 

 by covering plates or by pinnules, and so shut ofi' from the surrounding water? It 

 could not breathe properly in this condition, neither could it get its food. None of the 

 food particles which one finds so frequently in the alimentary canal of a Crinoid, 

 e.g., Eadiolarians, Foraminifera, Diatoms, &c., could enter the food-grooves of the arms 

 if they were closed by covering plates or by the apposition of the pinnules over them. 

 The habitual expansion of the arms is essential to the whole fife of a Crinoid, and 

 Prof. L. Agassiz has well described their movements in the living Rkizocrinus. " We 

 had the Crinoid alive for ten or twelve hours. When contracted the pinnules are 

 pressed against the arms, and the arms themselves shut against one another, so that 

 the whole looks like a brush made of a few long coarse twines. When the animal opens, 

 the arms at first separate without bending outside, so that the whole looks like an 

 inverted pentapod ; but gradually the tip of the arms bends outward as the arms 

 diverge more and more, and when fully expanded the crown has the appearance of 



