52 THE LEECH. 



1. The supra-oesophageal ganglia are a pair of closely 



apposed bodies in the dorsal wall of the anterior 

 end of ihe pharynx, immediately behind the jaws. 

 They supply the jaws, the eyes, and other parts of 

 the anterior end of the body. 



2. The nerve-collar is very small, and closely surrounds 



the anterior end of the pharynx. 



8. The ventral nerve-chain lies in the ventral sinus, which 

 must be slit open to expose it. It consists of two 

 halves very closely apposed in the median plane. 

 There are twenty-three pairs of ganglia, lying in 

 the first annuli of their respective somites. The 

 successive pairs of ganglia are much closer together 

 at the two ends of the body than they are along the 

 middle portion of its length. 



The first, or sub-oesophageal ganglia, are really 

 multiple, and give off five pairs of nerves. 



The last, or twenty-third, are also multiple, and 

 give several branches to the posterior sucker. 



From each of the other pairs of ganglia two pairs 

 of nerves are given off, supplying the corresponding 

 somite. 



F. Sense Organs. 



1. Eyes. The leech has ten eyes, seen as minute black dots, 

 arranged round the dorsal edge of the anterior 

 sucker. Each consists of a pigmented cup, filled 

 with rod-Hke bodies, and receiving a nerve at its 

 base. The eyes differ very little from smaller cup-like 

 segmental sense-organs found on the first annulus of 

 each segment throughout the body. Their structure 

 can only be made out by examination of microscopical 

 sections. 



III. EXAMINATION OF TRANSVERSE SECTIONS. 



Many points in the anatomy of the leech can only be 

 determined by examination of microscopical preparations, 



