268 CRINOIDEA. 



column was round ; but in Platycrinus it is oval or elliptical 

 (fig. 152). In the genera Pentacrinus (fig. 157) and Extra- 

 crinus the column is pentagonal in outline ; but much less 

 markedly so in the former than in the latter genus. The 

 joints articulate with one another by surfaces or facets which 

 are differently marked in different cases. In the Palaeozoic 

 forms, as in Platycrinus (fig. 152), the articulating facets are 

 marked by more or less numerous striae which radiate from 

 near the centre of the joint. In most of the Mesozoic genera, 

 on the other hand, as in Pentacrinus (fig. 157), the articu- 

 lating facets are united by crenated ridges arranged in a 

 pentapetalous figure. In many cases, as in Extracrinus and 

 Pentacrinus, the column is furnished with more or less 

 numerous " auxiliary " arms, or " side-arms," the function of 

 which is not altogether clear. The column increases in height 

 by the interpolation of new joints between the base of the 

 calyx and the highest articulation of the stem ; and each ar- 

 ticulation is pierced by a variously shaped perforation. Hence, 

 by the apposition of the successive joints there is formed a 

 tube the so-called " alimentary canal " of the older writers 

 which runs the entire length of the column. This canal is 

 most commonly round, but it may be pentapetalous, or it 

 may consist of four or five canals running parallel with and 

 around a central tube, into which they may or may not 

 open. This canal sends off diverticula into the side-arms 

 and the root-like processes of attachment, when these struc- 

 tures are present ; and it contains, in living forms, a vascular 

 axis (partly nervous in nature ?) which is connected superiorly 

 with a peculiar chambered organ situated in the base of the 

 calyx, and which doubtless serves to maintain the vitality of 

 the column and its appendages. 



The dorsal surface of the cup or " calyx " is composed of 

 a number of calcareous plates accurately fitted together. The 

 number and arrangement of these vary much in different 

 genera, and it will be sufficient to indicate here their gen- 

 eral disposition. 1 Eeposing directly upon the summit of the 



1 Different authorities have employed the various terms connected with the 

 plates in the calyx of the Crinoidea in such different senses that the subject 

 of the anatomy of the Crinoidal skeleton has been rendered very difficult to 



