MEDICINAL LEECH. 237 
These segments may be recognised externally by conspicuous. 
pigment spots (‘‘segmental papillee”), 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 papille,” and therefore in this region eight rings corre- 
spond to five segments. 
The penis is protruded on the middle véntral 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 
