CHAP. XVII.] 



ASTEROIDEA. 



439 



ASTEROIDEA. 



0. Symmetrical change in number of rays is common in some 

 of the forms. Asterias rubens and A. glacialis are frequently 

 seen with 6 or with 7 arms symmetrically arranged, and I have 



mouth 



&» = 



anus 



Fig. 133. Antedon rosacea having two arms abnormally divided. The figure A 

 shews the relations of the two abnormal arms, b 2 and e 1 , to the mouth and anus. B 

 shews the arm b 2 . (From Proc. Zool. Soc.) 



