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 body 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 segmeivtal 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). (V 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. 



