STRUCTURE OF THE LEECH. 215 



direct development, young leeches emerge and make for 

 the water. 



External Features. The leech usually measures from two to six 

 inches in length, and appears cylindrical or strap-like, according to its 

 state of contraction. The slimy body shows over a hundred skin rings, 

 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 unperforated hind sucker, above 

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



It is, however, necessary to consider the external characters in greater 

 detail. As already noted, the rings of the body are merely superficial 

 wrinkles ; it is therefore not difficult to realise that there may be doubt 

 as to their exact limits, and that the apparent number may differ accord- 

 ing as they are counted from the dorsal or ventral surface. According 

 to Whitman's precise investigations, 102 skin rings in all are represented, 

 and these correspond to 26 somites or true segments. 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 a pair of "eyes" occurs on each of the 1st, 2nd, 3rd, 5th, and 

 8th rings ; these are homologous with " segmental papillae," and there- 

 fore in this region eight rings correspond to five segments. 



Careful examination of the surface of the body will show further, the 

 swollen protrusion of the male organs on the middle ventral line between 

 rings 30 and 31, the aperture of the female organs five rings further 

 back, and also on the ventral surface seventeen pairs of small lateral 

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

 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. 



The Skin. 



The skin is so closely connected with the connective and 

 muscular tissue lying beneath that little can be seen of its 

 structure except in sections. 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 out- 

 ward passage of carbonic acid is readily effected. Opening 

 between the epidermal elements, but really situated much 

 deeper, are numerous long necked, flask shaped glandular 



