68 



THE MEDICINAI. LEECH 



C. The Nervous System. 



Cut through the anterior end of the pharynx and remove the alimentary 

 canal from- the bodJ^ On the anterior part of the pharynx are the two supra- 

 pharyngeal ganglia. They are connected by the circum-pharyngeal com- 

 missures to the ventral nerve- chain, which lies inside the ventral sinus and 

 which consists of twenty-three pairs of ganglia connected by two very closely 



. Anterior sucker 



Nephrostome 



. Supra- pharyngeal 

 Kflnelia 

 1st BflngHon 



Ist nephrldtum 



. n tst diverticulum 

 I) of crop 



- Nerve cord 



Lateral vesael 



Oviduct 



- Common duct 



Stomach- 



llth dtvertjculumi 

 of crop- 



-Vasa tifferentia 



■ 9th testis 

 Intestine 



17th nephridlum 

 Rectum 



23rd %anz\\on- 

 A.iiuff 



Fk;. 22. —A. The Leech [Hirudo medicinaUs)^ ventral view; B. Dorsal view of anterior 

 segments ; C. Dorsal view of posterior segments ; D. Nephridiura ; E. Female genital 

 organs (spread out) ; F. Dissection (made from dorsal- surface) to show the internal 

 structure. The alimentary canal has been cut open and the right half removed (as far 

 as the end of the crop), so as to expose the underlying organs. A.-E., after Borradaile ; 

 F., after Parker and Haswell. 



apposed connectives. The first or sub-pharyngeal ganglia are larger than .the 

 others and represent the fusion of the first five pairs of ganglia. The last 

 ganglia represent six or seven pairs fused together. Each ganglion or conjoint 

 pair of ganglia gives off nerves to supply the corresponding segment. 



D. Reproductive System. 



The ovaries are a pair of very small coiled bodies enclosed in small rounded 

 sacs, lying close to the mid-ventral line in the 11th segment. From each 

 sac a short oviduct passes inwards and backwards ; the right one passes under 



