THE CRINOIDEA 



119 



regularity is, however, often modified in the posterior interradius, 

 which is widened by the insertion of " anal plates," so called 

 because they afford room for the anus, and are continuous with 

 the plates supporting the anal tube when that organ is present. 



The Anal Plates of the Camerata appear as a median line 

 splitting the posterior interbrachials, and forming as it were a 

 sixth ray to the cup. They are rarely developed in forms in 

 which the anus is central or comparatively small ; a slight 

 enlargement of the posterior calycal elements then sufficed. They 

 are, therefore, regarded as supplementary pieces developed as 

 occasion arose in the position 

 where they are found. The anal 

 tube is an outgrowth of the pos- 

 terior interambulacrum, and is, in 

 Reteocrinidae, Glyptocrinus, and sim- 

 ilar forms, supported by a dorsal 

 line of ridged plates continuous 

 with the anals (Fig. XXV.). The 

 ridge is connected with the ridges 

 that unite the posterior basal to 

 the right and left posterior radials, 

 and this indicates that an axial 

 cord passed up it to govern the 

 motions of the tube. In later 

 Camerata, where the interradii and tegmen are less flexible, this 

 differentiation disappears. 



The anals of the Inadunata and Flexibilia (Fig. XXVI.) have 

 been much discussed (see summary in Bather, 1890 and 1899 ; and 

 AVachsmuth & Springer, 1897). At least one of them, the 

 radianal (RA), is admittedly a primary, abactinal, radial element, 

 being in fact the modified lower half of the right posterior radial 

 (r. post. Ri). By the introduction of other plates, and notably 

 one special anal (x) into the posterior IR, the r. post. Rs is pushed 

 to the right, so that RA comes into contact with x, and helps in 

 the widening of the area and the support of the anal tube. The 

 theory, originated by Wachsmuth and Springer, that the tube is a 

 modified arm, has since been rejected by them. The less extreme 

 view that the dorsal median line of ossicles supporting the tube 

 represents the proximal left ramus of the right posterior arm, up 

 which the interambulacral peristome around the anus gradually 

 stretched, has the following facts among others in its favour : 

 The tube is admittedly in close connection with the right posterior 

 ray (Fig. XXVII.) ; it is up this side that the rectum passes ; in 

 locrinus, Merocrinus, and C'astocrimts, and, to a less extent, in 

 Heterocrinus, Ectenocrinus, and Ohiocriuus, the proximal plate of 

 this median row rests on r. post. R ; in locrinus the articulation 



FIG. XXV. 



Glyptocrlwsu ilecailactyltts. 1, from pos- 

 terior interradius ; 2, another interradius. 

 The ridges marking the rays are clearly 

 seen, also ar, the anal ridge. (1, after 

 Wachsmuth & Springer. J, after Meek.) 



