THE MEDICINAL LEECH 13 



the circulatory system rather than of the excretory system (Bhatia, 

 1938). It is totally enclosed within a blood-filled sinus which lies 

 on top of the testis sac and consists of a central capsule or reservoir 

 which is studded with many small ciliated funnels. The reservoir 

 is the site of formation of corpuscles of the coelomic circulatory 

 system (see p. 15) and the cilia of the funnels beat outwards, 

 wafting the corpuscles into the blood. The initial lobe of the 

 nephridium lies close to this ciliated organ, but has no connexion 

 with it. From the testis sac a winding intra-cellular canal may be 

 followed to the main body of the nephridium. Here it joins with 

 a network of intra-cellular canals, and these eventually lead to an 

 intercellular canal which loops several times round the nephridium 

 before running to the vesicle. All the glandular part of the nephri- 

 dium arises from a nephridioblast cut off early from ecto-mesoderm 

 but the vesicle and its duct to the exterior are ectodermal. The 

 whole nephridium is closely invested with branches of the blood 

 sinus system and excretory products are obtained from these rather 

 than from the ciliated organ. 



The first six, and the last pair of nephridia are not associated 

 with testes. They do not have a ciliated organ, and the initial lobe 

 ends blindlv a little distance from the ventral nerve cord. 



6. Nervous System 



The central nervous system consists of a paired ventral nerve 

 cord connecting 34 paired ganglia. Of these, six are in the head 

 region, 21 are spaced along the ventral cord in the body and seven 

 are fused into a terminal mass in the posterior sucker. This 

 arrangement is shown in Fig. 3b. A typical ganglion of the ventral 

 chain consists of six capsules containing nerve cells, two median 

 ventral and the rest latero-dorsal in position, arranged round a 

 mass of nerve fibres. In the anterior and posterior ganglionic 

 masses there are also six capsules to each segmental ganglion and 

 these provide the basis for analyzing each mass. 



In the head region there is a (paired) supra-pharyngeal ganglion 

 or brain lying dorsal to the pharynx at the level of segment VI 

 and a (paired) sub-pharyngeal ganglion connected to the brain by 

 peri-pharyngeal connectives. The distribution of cell capsules 

 in this mass was determined by Livanow (1904) and is shown in 



