134 



LEECHES 



annelids have retained a simplified form of ciliated larva the 

 glossiphoniids have progressed further and suppressed it entirely. 

 On the other hand it is among the Glossiphoniidae that we find 

 a pattern of cleavage most closely resembling that of the poly- 

 chaetes and Schmidt (1944) takes this to mean that the Glossi- 

 phoniidae are a primitive group of leeches and that they never 

 evolved the thick-walled cocoon and its attendant larval adapta- 

 tions. The peculiarities of the cleavage process in Gnathobdellae 

 and Piscicolidae are seen by Schmidt as preliminary stages 

 in the development of special larval organs. 



9. Evolutionary History of Leeches 



While we are on the subject of phylogeny it is worth while to 

 draw together the various lines of evidence for the relationships of 

 the various groups of leeches. Wendrowsky (1928) determined 

 the chromosome number in various species. These are listed in 

 Table 8. They suggest that the primitive diploid number is 16 



Table 8 



and that Theromyzon^ which has a primitive pattern of develop- 

 ment is also primitive in its chromosome number while Piscicola 

 which has a specialized mode of development appears to be a 

 tetraploid. 



Another line of evidence is from the number of annuli per 



