264 



CRINOIDEA. 



have been a modification of this arrangement. In these 

 forms, as in Actinocrinus (fig. 153), the arms have much the 

 structure of those of the recent Crinoids, and are deeply 

 grooved on their ventral surfaces. The ventral surface of 

 the calyx, however, exhibits no central aperture, but only a 

 proboscidiform tube, which arises from one of the inter-radial 

 spaces (i.e., one of the intervals between two of the arms). 

 This tube is often of great length, and a good deal of con- 

 troversy has taken place as to its nature. Without entering 

 into the conflicting views upon this subject, it may be stated 

 that the preponderance of authority is overwhelmingly in 

 favour of the view that this " proboscis " is an anal tube, 



Fig 153. Calyx 

 of A ctinocrinus ro- 

 tundus. 



Fig. 154. Calyx 

 of Actinocrinus Ko- 

 nincki. 



Fig. 155. Calyx of A. 

 Verneuillanus. The arms 

 are wanting, and the aper- 

 tures at their bases are seen. 



having the vent at its extremity, all analogies based upon 

 recent forms bearing out this view. In the ancient types 

 in question, at any rate, the grooves on the ventral surfaces 

 of the arms are certainly not continued over the ventral 

 surface of the calyx, but, on the contrary, stop short at the 

 bases of the arms. Their further course was long a mystery ; 

 but it is now known that they are continued 'below the ven- 

 tral surface of the calyx as a series of covered passages or 

 tunnels, the external apertures of which are placed at the 

 points where the arms spring from the disc (see figs. 153- 

 155). These covered channels are simply roofed over by 

 the calcareous integument of the calyx ; and they converge 

 to a central point in the middle of the ventral surface of the 

 disc. Here is placed the mouth, concealed by the calcareous 

 plates of the perisome. In this point of their structure, 



