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 living animal the food grooves are lined by cilia, which are kept in a 
continual vibratory motion so as to produce currents of water, 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 Paleozoic species, with perhaps a few exceptions. In the Camerata, and 
especially among the Platycrinide, 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- 
tegminal in the same genus. Those of the Platycrinide, asa rule, are tegmi- 
nal, those of the Actinocrinide 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 “ Cceliac ” canal. 
