vii PHYLUM TROCHELMINTHES 339 



arrangement. The excretory system takes the form of ciliated tubes (ex}, closed 

 internally, and showing a tendency to metamerism : these extend into the head. 

 The sexes are distinct : the male has a protrusible penis, directed ventrally. 

 There is no metamorphosis. 



There seems to be some reason for believing that Dinophilns and the 

 Histriobdellea may help to bridge over the interval between the Trochelminthes 

 and the higher segmented worms or Ammlata. In this connection the 

 Echiuoderidse, which were noticed in an appendix to the last Section (p. 319), 

 have also to be kept in view. 



z 2 



