HIRUDINEA. 



515 



Sb 



or sunk in the cutis, or even amongst the muscles. The clitellar 

 glands are specially enlarged modifications of the ordinary skin gland, 

 and are found deeply situated in the clitellar region ; they secrete a 

 clear viscid substance which quickly hardens outside the bojiy and 

 is used to form the cocoons when the eggs are laid. Within the 

 muscles there is a connective tissue, in which the various organs are 

 embedded. These organs we may now proceed to consider. 



The central nervous system has the typical Annelidan character 

 of two ventral cords closely approximated, but diverging from one 

 another in front to pass round the oesophagus. There is a bilobed 

 swelling dorsal to the alimentary canal constituting the supra- 

 oesophageal ganglion, and supplying the 

 cephalic sense-organs; and a series of 

 swellings on the ventral cords, each 

 of which gives off two pairs of nerves. 

 These ventral ganglia are segmental in 

 their arrangement, but are partly fused 

 anteriorly and posteriorly. Thus in the 

 suboesophageal ganglion and circum- 

 oesophageal commissures five (or six) 

 ganglia can be made .out (Fig. 416); 

 these belong to the first five (or six) 

 segments of the head region. The last 

 ganglion, which supplies the sucker, 

 consists of seven (or six) ganglia 

 fused, and between these two composite 

 ganglia there are twenty-one separate 

 ventral ganglia* (Fig. 424). There is 

 an unpaired median longitudinal nerve 

 passing from ganglion to ganglion be- 

 tween the two halves of the ventral 

 cord and a system of visceral nerves 

 which was discovered by Brandt. The 

 latter consists of an intestinal nerve, 

 which arises from the brain and runs 

 close to and above the ganglionic chain; 

 it sends branches to supply the caeca 

 of the stomach. Three ganglia, which 



FIG. 416. Anterior end of Hirudo 

 (after Leydig). G cerebral ganglion, 

 with suboesophageal ganglionic 

 mass ; Sp visceral nerves ; A eyes ; 

 Sb sense-organs. 



* According to Bourne's enumeration there are 23 ventral ganglia, but he 

 counts the suboesophageal and posterior ganglion as single ganglia ; if the 

 former be allowed at 5 and the latter at 7, it makes the total number 33. 



