ANTEDON 375 



comparatively small cavity, is pentagonal, with rounded 

 and slightly upturned angles (fig. 19). In the peripheral 

 portion of the bottom of the cavity may be seen the 

 inner ends of a number of minute canals, through which 

 nerve cords pass to the cirri (PI. V., fig. 52; PL VI. , 

 fig. 59). The central portion of the aboral face of the 

 plate is flat or only slightly convex (PI. I., fig. 20). The 

 peripheral portion is wholly occupied by a number of 

 sockets for the articulation of the cirri. Those which 

 border the flattened central portion of the plate have a 

 nearly circular outline, while the more peripheral ones 

 become more or less angular, owing to close approxima- 

 tion. At the bottom of each socket there is a minute 

 papilliform elevation, perforated in its centre by the 

 external opening of one of the canals already mentioned. 

 The oral face of the plate is nearly flat. Shallow grooves 

 traverse it at the interradial angles from the periphery to 

 the margin of the cavity, marking it out into five equal 

 and radial segments (fig. 19). 



The Cirri. — These vary in number from twenty to 

 nearly forty. Each cirrus (PI. I., fig. 3; PL III., fig. 38) 

 is composed of from ten to eighteen calcareous joints, the 

 terminal one being in the form of a claw, immovably 

 articulated with the penultimate joint. The proximal 

 and next succeeding joints are cylindrical and shorter 

 than the remainder, which are compressed from side to 

 side and slightly constricted about the middle of their 

 length, the latter character becoming less marked, almost 

 to disappearance, in the terminal joints. A minute canal, 

 continuous with one of the canals of the centro-dorsal 

 plate, traverses the whole series of joints and ends 

 blindly in the terminal claw (fig. 38, ax. en.). All the 

 joints, except the penultimate and terminal ones, are 

 united by elastic ligaments, and the interarticular spaces 



