64 THE VOYAGE OF H.M.S. CHALLENGER. 
area as possible of the ciliated ambulacral grooves on their 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 Cyathocrinus would 
be as effective 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 Cyathocrinus. But for which of 
them are the covering plates of the arm-grooves at all adapted, and how far can these 
plates be considered as repetitions 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 déath (Pl. XIII. fiz. 16; Pl XVII. fig. 7; Pl XXVIL fig. 12; Pl XXXIX. 
fig. 12; Pl. 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. VIIIa. fig. 5—cp. Pl. XVII. figs. 2, 8, 9; 
Pl. XLVIL. 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 (Pls. XVIII, XIX.,XXV., XXVIIL., 
XXXL, XLV., XLIX., LIT.) ; while in other cases they are more or less expanded, as 
they were during life (Pls. XXXIV., XL., XLII.). Messrs. Wachsmuth and Springer argue, 
however, (1) because the arm-groove of the fossil Cyathocrinus 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 off from the surrounding water 
by the apposition of the pinnules when the arms were closed; therefore the covering 
plates of Cyathocrinus 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 off 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., Radiolarians, 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 life of a Crinoid, and 
Prof. L. Agassiz has well described their movements in the living Rhizocrinus. ‘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 
