36 THE CEINOIDEA CAMEEATA OE NOETH AMEEICA. 



the calyx by means of the amhdacral 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 anihulacral vessels, the ovarian tube, and the axial 

 canal. HhQ food-groove forms the upper passage. It is followed in descending 

 order by the suUeniacular canal, the genital canal, and the axial 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 Palaeozoic Crinoids permanently, and in the Antedon larva temporarily, 

 are mere grooves at the bottom of the ventral furrow, but in the mature 

 recent Crinoid, and in a few Pal^ocrinoidea, are separated from the furrow 

 by a limestone partition. 



The ambulacral plates consist of the ad-ambidacral or sidepieces, and the 

 covering plates, or Saimipldttchen ; 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 interamMacral plates occupy 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 iQxm perisomic plates is given to all plates which are originally devel- 

 oped from simple, cribiform films of hmestone. They comprise the inter- 

 radials and interaxillaries, the anals, and all ambulacral and interambulacral 

 plates. 



