100 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



by the covering plates, and frequently bordered by small side pieces ; the 

 former arranged alternately with each other and with the side pieces. In 

 the Hving animal the food grooves are lined by cilia, which are kept in a 

 continual vibratory motion so as to produce currents of w^ater, by means 

 of which any particles of food that happen to fall upon the grooves are 

 transmitted toward the mouth. Beneath the food groove lies a nervous 

 band, and beneath that a blood vessel, which in turn is followed by the 

 genital canal, and this by the subtentacular canal ; the genital canal, which 

 is quite small, occupying only the median portions. The subtentacular 

 canal, also known as the ambulacral canal proper, from which branches are 

 given off to the tentacles, communicates with the annular vessel situated in 

 the lip around the mouth. Beneath the ambulacra is the axial canal,* 

 which occupies the bottom of the arm grooves, frequently piercing the body 

 of the plates. This canal is connected with the chambered organ at the 

 lower part of the dorsal cup, and contains the axial cords, which, as now 

 generally admitted, control the movements of the arms and pinnules; while 

 the nervous apparatus beneath the food grooves has no connection with 

 the muscles, and no influence upon the movements of the skeleton. 



The ambulacra of fossil Crinoids are rarely observed, and their presence 

 is usually only indicated by the open grooves within the arm skeleton. In 

 some cases, however, the side and covering pieces of the disk, and occasionally 

 those upon the arms, are preserved. 



In all recent Crinoids the covering pieces are movable from the tips of 

 the pinnules to where they enter the mouth, but they are rigid upon the disk 

 in Palaeozoic species, with perhaps a few exceptions. In the Camerata, and 

 especially among the Platycrinidae, they are often heavier and larger than 

 the interambulacral plates -, while in other groups, and chiefly among Silurian 

 forms (Plate III. Fig. 11), they are quite small. The larger the plates, the 

 more irregular they are in their arrangement, and the smaller the most 

 regular. It is also noteworthy that the ambulacra may be tegminal or sub- 

 tegrainal in the same genus. Those of the Platycrinidse, as a rule, are tegmi- 

 nal, those of the Actinocrinidge generally subtegminal ; but also the opposite 

 is the case in genera of both groups. 



There is considerable variability in the extent to which the ambulacra 

 are exposed upon the surface. In the Camerata they never extend out to 

 the centre of the tegmen, their proximal ends being always hidden by the 



* This canal is also known as the " Dorsal " canal, and as the " Coeliac " canal. 



