MEDICINAL LEECH. 237 



These segments may be recognised externally by conspicuous 

 pigment spots (" segmental papillae"), which in the middle region of 

 the body occur on every fifth ring. In type, therefore, five rings 

 correspond to a segment, but at either end of the body the number of 

 rings is abbreviated. In the head region there is a pair of ' ' eyes " 

 on the ist, 2nd, 3rd, 5th, and 8th rings ; these are homologous with 

 "segmental papillae," and therefore in this region eight rings corre- 

 spond to five segments. 



The penis is protruded on the middle ventral line between rings 30 

 and 31 ; the aperture of the female duct lies five rings farther back. 

 Also on the ventral surface there are seventeen pairs of small lateral 

 apertures, through which a whitish fluid may be squeezed the openings 

 of the excretory organs. The skin of segments 9-11 is especially 

 glandular, and forms the so-called clitellum or saddle, the secretion 

 of which forms the cocoon for the eggs. 



Skin. Most externally lies the cuticle a product of the 

 epidermis periodically shed, as we have already noticed. 

 In this shedding some of the genuine epidermis cells are 

 also thrown off. These are somewhat hammer-like units, 

 with the heads turned outwards, while the spaces between 

 the thick handles contain pigment and the fine branches 

 of blood vessels. As the latter come very near the surface, 

 a respiratory absorption of oxygen and outward passage of 

 carbon dioxide is readily effected. Opening between the 

 epidermal elements, but really situated much deeper, are 

 numerous long-necked, flask-shaped glandular cells, secret- 

 ing the mucus so abundant on the skin. Underneath the 

 epidermis there is much connective tissue, besides yellow 

 and green, brown and black pigment. 



Muscular system and "body cavity. The muscular 

 system consists of spindle-shaped cells arranged externally 

 in circular bands like the hoops of a barrel, internally 

 in longitudinal strands like staves. Besides these there 

 are numerous muscle bundles running diagonally through 

 the body, or from dorsal to ventral surface, and there are 

 other muscles associated with the lips, jaws, and pharynx. 

 The body cavity, though distinct in the embryo, is almost 

 'obliterated in the adult leech, where the predominant con- 

 nective tissue has filled up nearly every chink. 



Nervous system and sense organs. The nervous system 

 mainly consists of a pair of dorsal ganglia lying above the 

 pharynx, and of a double nerve-cord, with twenty-three 

 ganglia, lying along the middle ventral line. The dorsal (or 



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