14 



LEECHES 



Fig. 6. It is seen that the first ganglion pair consists of six capsules 

 dorsal to the gut, the second consists of three capsules on each side 

 of the gut, while the remaining four pairs of ganglia form the 

 ventral mass. There is an important difference between this 



Supra-oesophagea 

 ganglion 



Sub-oesophagea 

 ganglion 



Nerve to prostomium 

 and first eye 



Sympathetic 

 nerve ring 



ve to eye 2 



ve to eye 3 



ve to eye 4 



Nerye to sensilloe on EZ" 



Nerve to eye 5 



Ner\/e to sensilloe on 'SL 



Fig. 6. Diagrammatic representation of the anterior ganglia 



of Hirudo in dorsal view showing segmental numbering of the 



ganglia, the capsules containing nerve cells and the roots of the 



anterior segmental nerves. After Livanow, 1904. 



arrangement and that found in the earthworms. In the latter the 

 cerebral ganglion is a single ganglion pair associated with the 

 prostomium and all the ganglia of the body segments are beneath 

 the oesophagus. In the leech one pair of segmental ganglia have 

 migrated round the pharynx and are closely associated with the 

 prostomial ganglia. This foreshadows the arrangement found in 



