60 



BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Aruil tube. — ^A flesliy conical tube, usually of considerable height, situated in one of 

 the interradial areas of the disk (the anal area) and bearing at its summit the 

 anal opening (see figs. 1.5-19, p. 67, and pp. 110 [7], 111). 



AtuiI X. — Tlie interradial situated between the two posterior radials, distal to and 

 to the left of the radianal if that plate is present . In all the recent forms anal x 



ORAL PINNULES 

 I Br (PRIM1BRACMS^ 

 CENTRO-DORSAL 



CIRRUS SOCKETS 



CIRRI 

 Fio. 1.— Lateral \nEW or a speciuen of Antedon adeiatica from Trieste; for the sake of srapuciTT the four arms 



ON THE side opposite THAT FIGURED ARE OMITTED. 



is exactly Uke the other four interradials, and these are always present if anal x 

 is present. In the recent crinoids anal x, if pei-sistont, gives rise to a post- 

 radialseries exactly resembUng those on the radials, becommg itself transformed 

 into a plate indistinguisable from a true radial. This is the cause of the forma- 

 tion of G-rayed variants, the sixth ray being situated between the two posterior 



