TERMINOLOGY. 35 
of the Camerata also have auxiliary anal plates, which may be present eyen 
when the special anal is wanting; they are interposed between the inter- 
brachials, following the median line of the posterior area. Another plate, 
the so-called “second anal plate” of American authors, which only occurs 
in the Inadunata and Articulata, is now called the radianal. It rests within 
the re-entering angle of two adjoining basals to the right of the first anal 
plate, and is the lower half of a bisected radial, which only in some genera 
assumes anal functions. 
Certain groups have interaxillary plates, which occupy the spaces within 
the axil of the distichals and palmars; the plates between the former are 
the iterdistichals ; those between the latter the énterpalmars. 
The ventral disk embraces the disk ambulacra, the mouth, and the anus. 
It is formed by the orals, the ambulacral and interambulacral plates. The 
posterior side of the disk in certain forms of the Inadunata, to which we 
have given the name Jnadunata Fistulata is extended upward into a large sac 
or tube, which is called the ventral sae or ventral tube. This tube is frequently 
perforated by pores or slits, which probably promoted respiration. Other 
forms of that group do not have these pores through the sac, but have 
a single, rather large, profusely perforated plate upon the disk proper, be- 
tween the sac and the mouth, — the so-called madreporite. The disk in most 
of the Camerata has small respiratory pores or slits near the arm bases, piere- 
ing the sides of the plates. In recent Crinoids, and probably in the Articu- 
lata generally, in which the pores penetrate the body of the plates, the 
perforated plates have received the name an-ambulacrals. 
The anus is interradial in position; its opening may be either central, 
subcentral, excentric or marginal; placed at the distal end of a tube, or 
opening directly through the disk. 
The “anal” tube must not be confounded with the “ ventral” tube of the 
Fistulata, which often does not contain the anus, but when it does, the 
opening is generally on the anterior side. 
The mouth occupies the centre of radiation, and is ¢egminal or subtegminal. 
If it is ¢egminal, the opening is surrounded either by the orals and the ends 
of the ambulacra, or, when the orals are absent, by interambulacral plates, 
which form a lip around it. If swbtegminal, it is completely closed, either by 
the orals or interambulacral plates, which form a roof over it. 
The ambulacra diverge from the mouth to the tips of the rays, following 
the ventral furrows of arms and pinnules. When subtegminal, they enter 
