36 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
the calyx by means of the ambulacral or arm openings at the upper edge of 
the dorsal cup; when tegminal, they follow the surface of the disk. They 
contain the food-groove, the ambulacral vessels, the ovarian tube, and the axial 
canal. The food-groove forms the upper passage. It is followed in descending 
order by the subtentacular canal, the genital canal, and the avial canal. The 
axial canal contains the axial cords, which communicate with the chambered 
organ at the dorso-central basin of the calyx. The axial canals, in most of 
the Paleozoic Crinoids permanently, and in the Axtedon larva temporarily, 
are mere grooves at the bottom of the ventral furrow, but in the mature 
recent Crinoid, and in a few Palzeocrinoidea, are separated from the furrow 
by a limestone partition. 
The ambulacral plates consist of the ad-ambulacral or side-pieces, and the 
covering plates, or Saumpliittchen ; the former, when present, constitute the 
outer, the latter the inner rows of the plates. The covering pieces form 
a roof over the food grooves, and are generally represented by two alternat- 
ing rows of small, more or less regularly arranged plates, which in all 
Crinoids are movable upon the arms and pinnules, but upon the disk only 
in those in which the mouth is exposed. In some of the Camerata the 
plates are so highly differentiated, that they have been regarded as alto- 
gether different structures, and were called radial dome plates. We retain 
this name as a conventional term for the large isolated plates of that group 
to distinguish them conveniently from the ordinary covering pieces. 
The orals consist of the five large interradial plates which surround the 
mouth or cover it, and are either symmetrical or asymmetrical. They are 
symmetrical when of nearly the same size and form; asymmetrical when 
the posterior plate is pushed in between the other four. In some species 
they occupy the entire ventral surface of the calyx; in others, only a com- 
paratively small space in the middle; or they may be completely resorbed 
in the mature individual. 
The inlerambulacral plates oceupy the spaces between the ambulacra, 
their main trunks as well as their branches. We also apply the term to the 
plates covering the ambulacra, and to those encroaching upon them from the 
sides, as in many species of the Camerata in which the disk ambulacra are 
subtegminal or partly so. 
The term perisomic plates is given to all plates which are originally devel- 
oped from simple, cribiform films of limestone. They comprise the inter- 
radials and interaxillaries, the anals, and all ambulacral and interambulacral 
plates. 
