Manchester Memoirs, Vol. Ix. (1916). 15 



bilaterally symmetrical forms are provided with a more 

 powerful and a more elaborate muscular system, particu- 

 larly as regards the rachis than the radially symmetrical 

 forms, and this indicates that the assumption of the 

 bilateral symmetry is correlated with increased powers of 

 locomotion. 



Nor are we better informed as to the evolutionary 

 history of the group. The ancestral form may have been 

 a fixed sedentary colony or it may have been a floating 

 colony. For reasons I have given elsewhere I believe that 

 it was a floating colony, but in either case it was un- 

 doubtedly radially symmetrical. 



It seems to me that these studies on the structure of 

 the Pennatulacea afford a very interesting and instructive 

 example of the way in which variable or plastic characters 

 may become less variable or rigid as we pass from the 

 radial to the bilateral symmetry and of the way in which 

 the increasing rigidity of certain characters leads in some 

 cases to the differentiation of the discontinuous groups 

 which are recognised as species. 



