CRINOIDEA. 



121 



There is no doubt that it is by the above arrangement that 

 the living Crinoids obtain their food, and the mechanism seems 

 to have been essentially the same in many extinct species. In 

 the Palaeozoic Crinoids, however, there seems to have been a 

 modification of this arrangement. In these forms, as in Actino- 

 crinus (fig. 75), the arms have much the structure of those of the 

 recent Crinoids, and are deeply grooved on theirventral 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 controversy 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 in favour of the 

 view that this " proboscis " is an anal tube, having the vent at 



Fig. 74. Calyx of 

 Actinocrinus rotundus. 



Fig. 75. Calyx of 

 Actinocrinus Koniticki. 



Fig. 76. Calyx of A. 

 Verneuillanus. The 

 arms are wanting, 

 and the apertures at 

 their bases are seen. 



its extremity. Good observers, however, regard it as discharg- 

 ing the functions of both the mouth and the anus. Be this as 

 it may, 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 ventral 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. 74-76). These covered channels are either 

 simply roofed over by the calcareous integument of the calyx, 

 or are rarely excavated in its walls ; and they converge to a 

 central point in the middle of the ventral surface of the disc. 

 Here, it is believed, is placed the mouth, concealed by the 

 calcareous plates of the perisome. From the known function 

 of the brachial grooves in the living Crinoids, this view would 



