HIRUDO MEDICINALIS LINN^US 109 



the following sixteen pairs are situated on the second annulus 

 of the successive segments. The twenty-third segment is there- 

 fore the last segment possessing nephridiopores. A nephridium 

 is a coiled tube usually with an open funnel; but in the case 

 of the leech the funnel which lies in a lateral branch of the 

 ventral sinus is closed. The funnel of the sixth to fifteenth 

 nephridia is in close contact with the testes and this is the 

 reason why the first part of the nephridium is called the testis 

 lobe. The largest part of the nephridium is formed by the 

 middle or glandular lobe which forms together with the so- 

 called apical lobe a closed, compressed ring and is connected 

 with the vesicle by a thin vesicle duct. Both the apical and 

 middle as well as the testis lobe are perforated by a compli- 

 cated, ramified system of canals. The vesicle is the last part of 

 the nephridium and is connected with the nephridiopore by a 

 very short duct. 



The nervous system consists of a brain or supracesophageal 

 ganglion which lies above the pharynx and just behind the jaws, 

 a subcesophageal ganglion underneath the pharynx and connected 

 with the brain by two heavy commissures, and a chain of twenty- 

 one pairs of ganglia inclosed in the ventral sinus, one pair of 

 ganglia for each segment except in the case of the last or twenty- 

 first pair which represents the fused neuromeres of the last 

 seven segments transformed into the posterior sucker. The 

 brain together with the subcesophageal ganglion represents the 

 fused neuromeres of the anterior six segments. The sympathetic 

 nervous system is represented by a median nerve which runs 

 above the nervous chain along the wall of the stomach. 



Reproductive system. The leeches are true hermaphro- 

 dites. The male reproductive organs in the medicinal leech 

 consist of nine pairs of testes situated beneath the stomach in 

 the ' thirteenth to twenty-first segments. There are two lon- 

 gitudinal vasa deferentia connected with the testes by short 

 vasa efferentia. Anteriorly each vas deferens forms a convo- 

 luted seminal vesicle. The next portion of each duct, connecting 



