NO. IO PHYLOGENETIC STUDY OF RECENT CRINOTDS CLARK 3 1 



Subsequently this simple type of arm became modified through 

 the interpolation between the arm base and the radial of the so-called 

 IBr series, a pair of ossicles which is in reality a more or less perfect 

 reduplication of the radial (corresponding- to the IBr 2 ) and the 

 infraradial (corresponding to the IBr x ). 



The presence of IBr series is rendered possible only by a very 

 considerable reduction in the development of the calyx. The arms 

 arise from the border of the disk, and are outgrowths of which the 

 dorsal skeletal structures are derivatives from the skeletal structures 

 of the sides of the calyx cup, while the ventral structures, the ambu- 

 lacral grooves, water, blood, muscle and ventral nerve systems, are 

 outgrowths from the corresponding structures on the disk which 

 have extended themselves outward over the dorsal skeletal structures 

 as a support. Being in part derived from the lateral body wall and 

 in part an outgrowth from the ventral surface, the arms necessarily 

 must maintain their original position on the edge of the disk. In 

 the reduction of the calyx from the primitive condition of a cup 

 entirely enclosing the visceral mass dorsally and laterally to the 

 form of a small cap covering only the dorsal pole of the visceral 

 mass, or of a platform upon which the visceral mass rests, the arms, 

 as much a part of the disk as of the dorsal skeletal structure, are 

 unable to maintain their original connection with the now greatly 

 reduced radials. The growing gap between the arm bases and the 

 radials is filled not by a dorsalward extension of the arm bases, but 

 by the formation of an entirely new pair of plates, the IBr series, 

 between the radials and the arm bases, which serve to maintain the 

 connection, and which are in origin and in structure a more or less 

 perfect reduplication of the now atrophied radial and infraradial 

 (or possibly infrabasal). The presence of IBr series is therefore 

 a certain indication of the suppression of other more extensive skele- 

 tal structures, and is therefore an indication of specialization through 

 suppression. In this respect the presence of IBr series is of the same 

 significance as the presence of cirri, which always indicate and accom- 

 pany a suppression of development in the column. As the specializa- 

 tion of the column through suppression of its growth is correlated 

 with the specialization of the calyx through suppression of its devel- 

 opment, it is only natural that we should find the development of 

 cirri more or less closely correlated with the development of IBr 

 (and additional comparable) series. 



