474 



AUSTRALIAN HIRUDINEA, III.j 



From the above data I conclude that the primitive stock from 

 which the Glossiphonid forms were derived, was originally pro- 

 vided with a penial structure, and that the genital apertures 

 were separated by a number of annuli, the pores being situated 

 probably in successive somites. 



G. hettroclita 



Species with pores sepaiatedx 

 by two annuli. 



Species with pores separated 

 by one annul us. 



Castle's elaborate and complete studies on the anatomy of the 

 North American Rhynchohdellidce, render it possible to construct 

 a phylogenetic tree, showing the position of the species in regard 

 to one another; and the above is, in the main, in agreement with 

 his conclusions on fundamental grounds. The study of the posi- 

 tion of the genital pores enables one to decide the points at which 

 the two side-groups left the central stem. There has been a 

 stimulus urging the approach of the genital apertures. In the 

 concolor-gvoM"^ we see a much nearer approach to other leeches in 

 general in regard to the position of the pores, this group leaving 

 the main stem, and being characterised by (1) three pairs of eyes, 

 (2) seven pairs of lobed crop-diverticula, (3) a rough papuliferous 

 integument, (4) asymmetrically arranged egg-clusters, and (5) 

 genital apertures separated by two annuli. Before this group 

 had left the main stem, sufficient time had probably elapsed to 

 permit of the shifting back of the male pore so that it now lay 

 between somites xi. and xii., instead of being situated further 

 forward in somite xi.(very probably between annuli 2 and 3 of 

 somite xi.). Along the main stem there had been a further 



