REPRODUCTION AND DEVELOPMENT 



135 



segment. Oligochaetes have one annulus per segment and leeches 

 pass through the uniannulate condition during development so it 

 is reasonable to infer that a small number of annuli is a primitive 

 feature. Once again it is the Glossiphoniidae with three annuli per 

 segment which appear to be the primitive group. Next come the 

 Gnathobdellae with five as the typical number and lastly the 

 Piscicolidae where more than five is common {Piscicola has 

 fourteen). 



It seems unlikely that the Hirudinea as a whole are polyphyletic 

 for it would be difficult to explain how more than one line of 

 evolution arrived at 33 segments as the optimum number. If we 

 therefore assume a common ancestral stock it is obvious that there 

 was an early and fundamental divergence between the Rhynchob- 

 dellae and the Gnathobdellae according to whether the method 

 of sucking blood was by a proboscis or by jaws. Within the 

 Rhynchobdellae there is abundant evidence for regarding the 



Piscicolidae 



Glossiphoniidae 



Erpobdellidae 



Hoemadipsidae 

 Hirudidae 



Rhynchobdellae 



Gnathobdellae 

 Acanthobdeilae 



Ancestral oligochaete stock 



Fig. 85. Diagram illustrating the evolution of the main 

 families of leeches from the ancestral oligochaete stock. 



Glossiphoniidae as primitive and the Piscicolidae as special- 

 ized. In the Gnathobdellae it is likely that those forms with well 

 developed jaws are primitive and that those which have become 

 carnivorous have done so secondarily. Using this evidence it is 

 possible to construct the evolutionary tree illustrated in Fig. 85. 



