50 THE LEECH 



3. The ventral nerve-chain Ues 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 iirst 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 pair, or sub-cesophageal ganglia, are 

 much the largest, and give off five pairs of nerves. 



The last, or twenty-third pair, are also large, 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, which are visible to the 

 naked eye as minute black dots, arranged round the 

 edge of the anterior sucker. Each consists of a 

 pigmented cup, filled with rod-like bodies, and 

 receiving a nerve at its base. The eyes differ very 

 little from smaller cup-like tactile bodies found dis- 

 tributed over the anterior part of the body. 



Their structure can only be made out by exa- 

 mination of microscopical sections. 



III. EXAMINATION OF TBANSVERSE SECTIONS. 



Many points in the anatomy of the leech can only- be 

 ■determined by examination of microscopical preparations ; 

 and for this purpose transverse sections of the body are the 

 most instructive. Of these half a dozen or more should be 

 examined, as the appearances will necessarily differ according 

 -to the particular organs and parts through which the section 

 happens to pass. 



The following description is a general one, and will apply 

 -to any section through the middle half of the body. 



