HIRUDINEA OR DISCOPHORA. 209 



its dorsal surface is beautifully marked with longitudinal pigmented 

 bands, while the ventral surface is mottled irregularly ; the suctorial 

 mouth is readily distinguished from the imperforated hind-sucker, above 

 which, on the dorsal surface, the alimentary canal may be seen to end. 



According to Whitman's precise investigations, there are 102 skin- 

 rings and 26 somites or true segments. These segments may be recog- 

 nised externally by conspicuous pigment spots (" segmental papilla; "), 

 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 a pair of "eyes" occurs on the 1st, 2nd, 3rd, 5th, and 8th 

 rings; these are homologous with "segmental papilke," and therefore 

 in this region eight rings correspond 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 further 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 

 carbonic acid is readily effected. Opening between the 

 epidermal elements, but really situated much deeper, are 

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

 contents of which form the mucus so abundant on the skin. 

 Underneath the epidermis there is much connective tissue, 

 and 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. 

 14 



