VII PHYLUM TROCHELMINTHES 339 



airangemeut. The excfetorj' system takes the form of ciliated tubes (ex), closed 

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

 The sexes ar6 distinct : the male has a protrusible penis, directed ventrally. 

 There is no metamorphosis. 



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

 Histriobdellea may help to bridge over the interval between the Trochelminthes 

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

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

 have also to be kept in view. 



z 2 



