ANTEDON. 373 



portions into which the tegmen calycis is divided by the 

 ambulacral grooves there is a conical projection, the anal 

 fnnnel (PL II., fig. 24, an. fl.), at the apex of which the 

 anus (an.) is situated. The margin of the anal aperture 

 bears minute papillae. With the aid of a good lens a 

 number of minute pores may be detected in the tegmen 

 calycis. These are the external apertures of the ciliated 

 funnels of the water vascular system. 



Orientation. — When Antedon is at rest with the 

 mouth directed upwards and the anal interradius nearest 

 to the observer, it is customary to call the opposite radius 

 anterior. The four remaining radii are thus right 

 anterior, right posterior, left posterior, and left anterior. 

 In correspondence with this designation of the radii 

 the interradii are called right anterior, right postero- 

 lateral, posterior (or anal), left postero-lateral and left 

 anterior respectively. To maintain this orientation when 

 the animal is viewed from the aboral surface the posterior 

 (anal) interradius must be turned away from the observer. 



The Skeleton. 



The skeleton of the calyx is composed of a centro- 

 dorsal plate (PI. I, fig. 3; PI. V., fig. 52; PL VI., fig. 59, 

 cut. dr.), to the convex aboral surface of which the cirri 

 are articulated; five radial plates (PL V., fig. 52; PL VI., 

 fig. 59, rd.), which form a pentagon and rest upon the 

 plane oral surface of the centro-dorsal ; and five basal 

 plates which are fused together to form a single plate, the 

 rosette (PL I, figs 17 and 18; PL V, fig. 52; PL VI., 

 fig. 59, ros.), which roofs over a bowl-shaped cavity in the 

 centro-dorsal (PL I., fig. 19). To these must be added, as 

 contributing to the support of the visceral mass, the two 

 most proximal joints of the arms, called respectively the 

 first and second primibrachials (PL I., fig. 3; PL V., 



