THE MEDICINAL LEECH 



15 



the Arthropoda, where several segmental ganglia contribute to 

 the brain. 



There are seven peripheral nerves in the head region, and the 

 sense organs with which each is associated are indicated in Fig. 6. 

 The sympathetic ( = stomatogastric) nervous system consists of a 

 nerve ring lying on the wall of the pharynx just in front of the main 

 nerve ring, and a plexus of nerve cells and fibres on the walls of 

 the gut (Terio, 1950). It links with the central nervous system at 

 two points on the circum-pharyngeal nerve ring. 



7. Blood System 



While more primitive leeches such as Glossiphonia have distinct 

 blood vessels lying within the coelomic sinuses (Fig. 16), Hirudo 

 has completely lost all traces of blood vessels and the blood circu- 

 lates in coelomic sinuses, some of which have secondarily acquired 

 muscular walls. There are four main longitudinal sinuses: a dorsal 

 sinus above the gut, a ventral sinus containing the ventral nerve 

 cord and anterior and posterior ganglionic masses, and two lateral 

 sinuses (Fig. 7). The lateral sinuses have the muscular walls and 



Dorsal sinus 



Lalera 



nus 



Latero- lateral sinus 



o-ventral sinus 



containing nerve cord 



Fig. 7. Reconstruction of the coelomic sinus system of Hirudo 

 from one segment in the mid body region. Original. 



are responsible for circulating the blood. They give off three main 

 pairs of branches in each segment which were named by Gratiolet 



