MEDICINAL LEECH. 



319 



Digestion of blood by the common Leech. Stirling, Journal Anat. and Physiol. xvi. 

 1882. 



Vascular system and coelome. Gibbs Bourne, op. cit. p. 453 ; Jaquet, Mittheil. 

 Zool. Stat. Naples, vi. 3, 1885. 



44. MEDICINAL LEECH (Hirudo medicinalis), 



Dissected so as to show its nervous system. 



A PART of the pharynx with the jaws has been left in situ, and a 

 black bristle also passed down it through the oesophageal or nerve-collar. 

 The supra-oesophageal ganglion is seen above the pharynx, part of the 

 glandular and muscular walls of which have been removed to show it in situ. 

 It lies immediately behind the dorsally placed jaw, which is very visible in 

 this preparation. It is connected by commissures forming an extremely 

 narrow ring to the ventral chain of ganglia. This chain, counting the infra- 

 oesophageal ganglion as the first of the series, numbers twenty-three 

 ganglia in all. The infra-oesophageal ganglion and the second ganglion, 

 which is closely apposed to it, are to be seen with difficulty here. The first 

 ganglion, easily seen, is the third of the series. The longitudinal commis- 

 sures between the third and fourth, the fourth and fifth ganglia increase in 

 length, though they are shorter than those connecting the ganglia belong- 

 ing to the middle region of the body. The sixth ganglion is concealed 

 by the prostate gland at the base of the muscular penis. It is close to the 

 seventh ganglion. The ganglia at the posterior extremity of the body, 

 beginning with the nineteenth ganglion, are again closely aggregated to- 

 gether. The last ganglion is much larger than any of the series except the 

 first. 



It can be readily seen with the naked eye that nerves are given off 

 laterally from each ganglion. They are in reality paired, but one branch is 

 dorsal, the other ventral. There are no nerves given off between the gan- 

 glia in the Leech, as there appear to be in the Earthworm. But in the 

 Leech the ganglion cells are really aggregated in the ganglia, not scattered 

 along the whole cord as in the last-named worm. 



In front of the supra-oesophageal ganglion lie three minute ganglia closely 

 connected to it, one median and two lateral. The nerves given off by these ganglia 

 supply the three jaws. The nerves originating from the supra-oesophageal ganglia 

 supply the eyes and the goblet-shaped organs of Leydig. 



The infra-oesophageal ganglion, according to Leuckart, is composed of three 

 ganglia in the embryo which fuse in the adult. As figured by Leydig, it is composed 

 of two halves, right and left, connected by five transverse fibrous commissures. 



