TEANSVEBSE SECTIONS 49 



3. The ventral nerve-cliaiii lies in the ventral sinus, which 

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

 halves, very closely apposed in the median plane. 

 There are twenty-three pairs of ganglia, placed in 

 the first annuli of their respective somites. The 

 successive pairs of gangha 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 gangha two pairs 

 of nerves are given off, supplying the corresponding 

 somite. 



F. Sense Organs. 



1. Eyes. The leech has ten eyes, arranged round the edge 

 of the anterior sucker. These eyes, which are visible 

 to the naked eye as minute black dots, consist each 

 of a pigmented cup, fiUed with rod-hke bodies, and 

 receiving a nerve at its base. They differ very little 

 from smaller cup-like tactile bodies found distributed 

 over the anterior part of the body. 



Their structure can only be made out by exa- 

 mination of microscopical sections. 



III. EXAMINATION OF TEANSVEBSE 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. 



