CRINOIDEA. • 417 



— which, as previously noted, lodges the neuro-vascular axis of the stem. 

 The neuro-vascular tube of the stem sends off diverticula into the cirri 

 and root-like processes of attachment, when these structures are present. 

 It is very commonly round, but it may be pentapetalous. In some cases, 

 there are four or five canals running parallel with and around a central 

 tube, but in this case the function of the outer circle of tubes is uncertain. 



Fig. 290. — Extracrinus briaroides, from the Lias, showing the crown of irms, and the column 

 with its side-arms. The small figures show the stem-joints of Extracrmus subangularis. 



The column carries at its summit the cup-shaped, pyriform, or 

 bursiform body of the animal, or "calyx" (fig. 291). As the 

 column is produced from the aboral pole of the animal, it is the 



VOL. I. 2D 



