TRANSVERSE SECTIONS. 61 



The first, or sub-cesophageal 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-like 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, 

 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 region of the body. 



A. Shape. 



A transverse section of the body of a leech is oval in out- 

 fine, the transverse diameter being the greater, and the ventral 

 surface being flatter than the dorsal. 



B. The Integument. 



1. The cuticle. The most superficial layer is a thin struc- 

 tureless elastic cuticle, which is secreted by the 

 underlying^epidermis, and is shed and renewed at 

 frequent intervals during fife. 



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