ON SOME PLYMOUTH HOLOTHURIANS. 



233 



collar is shown in Fig. 13, from which it will be seen that it is very similar 

 to the portion figured by Marenzeller (18) : the connecting pieces are 

 not calcified in the smaller forms. The polian vesicle is single and very 



Fig. 13. — External view of the calcareous collar of Thyone raphaniis Diib. and Kor. 

 drawn from a glycerine-jelly preparation after separating the ventral piece 

 from the adjacent right radial piece : the polian vesicle and the stone canal are 

 shown attached to the collar* ( x 9). 



long, and longer in the larger than in the smaller specimens ; in preserved 

 specimens it may be dilated at the proximal or the distal end, or at both 

 ends. 



* The constituent pieces of the collar are somewhat delicate, so that drawings from 

 potash preparations are liable to be inaccurate, inasmuch as the middle portion of the 

 radials is very thin and might easily be abraded to give such a figure as is drawn by 

 Barrois for T, poucheti (loc. cit. ). 



