TEEMINOLOGY. 35 



of the Camerata also have auxiliary anal plates, which may be present even 

 when the special anal is wanting ; they are interposed between the inter- 

 brachials, following the median Hne 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 interdistichals ; those between the latter the interpalmars. 



The ventral disk embraces the disk ambulacra, the mouth, and the anus. 

 It is formed by the orals, the amhulacral and interambulacral plates. The 

 posterior side of the disk in certain forms of the Inadunata, to which we 

 have given the name Inadunata Fistulata is extended upward into a large sac 

 or tube, which is called the ventral sac 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 prajjer, be- 

 tween the sac and the mouth, — the so-called madreporite. The disk in most 

 of the Camerata has small respirator?/ pores or slits near the arm bases, pierc- 

 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-amlulacrals. 



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 '^ anaV tube must not be confounded with the " ventraV 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 tegminal or suhtegminal. 

 If it is tegminal, 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. li sultegminal, it is completely closed, either by 

 the orals or interambulacral plates, which form a roof over it. 



The amhilacra diverge from the mouth to the tips of the rays, following 

 the ventral furrows of arms and pinnules. When suhtegminal, they enter 



