THE HIRUDINEA. 



191 



sure connecting it with a ventral cord on which ganglia, cor- 

 responding in number with the somites of the body, are de- 



Fig. 50.— A diagrammatic view of the arrangement of the principal vessels of the 

 leech (ffirudo medicinal is), after Giatiolet. The inner surface of a portion of 

 one-half of the body is depicted: a, a, the ventral trunk; e, e', e", tbe lateral 

 trunk and its branches ; f, /', the dorsal trunk and its branches ; g, the slender 

 transverse trunks which branch out at each end; h, i, the transverse ventral 

 branches of the lateral trunk ; k, I, the branch to the testis (c), and the segmental 

 organ (d) ; m, branch from the dilatation on the testis to the parietal plexuses; 

 b, b, vas deferens. 



veloped. In 3falacobdelia, these cords are lateral and wide 

 apart, but, in all the other Hirudinea, they come close to- 

 gether behind the mouth, and occupy the middle line of the 

 ventral face of the body. In the Leech, according to Leuck- 

 art, there are originally thirty pairs of post-oral ganglia, but 

 the seven posterior and the three anterior pairs coalesce, so 

 that only twenty-three pairs are distinguishable in the adult. 

 Nerves are given off to the pharynx and intestines, and the 

 former develop special ganglia. 



Simple eyes are usually present on the anterior or oral 

 segment, and receive nerves from the supracesophageal gan- 

 glia. In the Leech these eyes are situated in the first three 

 segments. Cup-shaped depressions of the integument of the 

 anterior segments of the body, lined by peculiar glassy cells 



