THE IIIRUDINi:.\ 



191 



sure connecting it with a ventral cord on which ganglia, 

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



Fio. 50. A diagrammatic view of the arrangement of the principal vescel* of the 

 leech (Hirudo mi<H<-in<ilix). after Giatiolet. The inner fnrface of a portion of 

 one-half of the body is depicted: <i. , tin- vcntr.il trunk; , ',", the lateral 

 trunk and its brandies ; /, /', the dorsal trunk and its branches ; g, the n|< 

 transverse trunks wliirli branch out at each ml ; //, /. the t: > nirti 



branches of the lateral trunk; k % I, the branch to tin- te-tis (a), and the scemental 

 or;;;m (</) ; m, branch from the dilatation on the te<tis to the parietal plexuses; 

 6, 0, vas deferens. 



veloped. In MalacobdeUa, 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 lino of tin* 

 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 BUprftoeaophageal 

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

 segments. Cup-shaped <1 -pi rssions of the integument of the 

 anterior segments of the body, lined by peculiar glassy cells 



