4 LEECHES 



nerves. On critical examination (Mann, 1953a) it appears that the 

 method of Castle and Moore is the correct one, for the inter- 

 segmental boundaries then coincide with the intersegmental septa 

 which are present in the embryo but are lost in the adult leech. 



The earlier method of delimiting segments led to disagreement 

 about the total number of segments present as well as about their 

 exact limits but analysis of the nervous system has led to the 

 conclusion that there are thirty-four pairs of ganglia in all, of 

 which the first is presumably homologous with the pre-oral 

 ganglion of other annelids and belongs to the prostomium. The 

 second ganglion pair is situated on the circum-oral commissures 

 or is fused with the first, the next four are fused into a post-oral 

 ganglionic mass, twenty-one (normally) are distributed along the 

 ventral nerve cords, and the last seven are fused into the ganglionic 

 mass of the posterior sucker. The descriptions of leeches which 

 follow are based on this analysis. 



