The epidermis or hypodermis of the medicinal Leech consists of mallet-shaped cells about 1/1500 of an inch long. The heads of the mallets are placed superficially beneath the cuticle. The handles are consequently separated by spaces, into which processes of pigmented connective tissue cells with capillaries make their way. The nucleus lies in the handle of the mallet. The hypodermis cells vary much in character in different Leeches, and in Rhynchobdellidae the pigment cells and capillaries do not always intrude between them.
From the hypodermis cells are produced numerous unicellular glands. In Hirudo these glands pass into the dermis, and their ducts are consequently long. Those of the general body-surface appear to be simply mucous glands. The more deeply situated glands are (1) clitellar glands, which occur in the clitellar region in groups of four to five, the glandular part lying in the longitudinal muscle layer; (2) prostomial glands of doubtful function, but probably found only in Gnathobdellidae, with ducts opening round the edges of the buccal cavity, and their contents not staining with borax carmine; (3) salivary glands, which belong to the buccal cavity itself, with ducts opening on the ridges which bear the teeth, and with contents which stain with borax carmine.
The hypodermis cells are also modified to form sensory cells, with which nerves are continuous. In the head region are found the goblet-like bodies and the eyes, the visual nature of which is doubted by Carriere. Of the latter there are ten, two on the first and second annulus, and two on the first annulus of the three following somites. Each eye consists of a projecting cap of short non-pigmented hypodermis cells, covered by cuticle and containing at their outer ends small retractile bodies: of an external layer of pigmented cells, separated by a lamina (?) from a layer of nucleated clear cells with well-defined walls and protoplasm largely replaced by a vacuole, and lodging a corpuscle of unknown significance: and of a slender core of hypodermic cells as they appear to be from a comparison with the segmental papillae and from their histo-chemical properties (Whitman). The nerve enters at the side but near the base, and at its point of entrance there appear to be ganglion cells. The goblet-bodies and segmental papillae have a bulb-like thickening of the hypodermis cells, no pigmented coat, and relatively few clear cells. The former are exceedingly numerous (about sixty in number), especially on the anterior margin of the prostomium.
They are situated on branches of the same nerves as the eyes. The Leech is sensitive to light, but the prostomial region appears to be equally if not more sensitive to other impressions (touch, taste ?). The eyes of Clepsine differ from the eyes of Hirudo, and consist, according to Carriere, of a semi-globular cup of large pigmented retinal cells, containing a number of transparent and smaller cells. They are situated in the muscular coat. The eyes of Nephelis appear to be similar; those of Piscicola, a Leech in which they occur on the posterior as well as the anterior sucker, appear to be more simple, judging from Leuckart's figures. The visual nature of these eyes seems certain.
The sub-hypodermic tissues consist of a connective jelly-like matrix imbedding nucleated corpuscles with very fine branches, and more or less pigmented, as well as 'vaso-fibrous' tissue: The latter consists of nucleated branched cells, containing a plentiful supply of pigment granules. Their branches sometimes become tubular, and the nuclei project into the lumen, and even drop into it. These cells occur in all parts of the body, and their processes pass out between the hypodermic cells. According to Professor Lankester, the tubular cells may become continuous with the thin-walled capillaries, in the cavities of which free nuclei may sometimes be detected. The botryoidal or so-called 'hepatic' tissue, which surrounds, but not in immediate contact, the walls of the alimentary tract appears to be merely a modification of the same tissue. It consists of largish vessels with cellular walls. The individual cells are swollen up, nucleated, and pigmented. The vessels thus formed partly end caecally, partly form a plexus continuous with the ordinary thin-walled capillaries. The corresponding pigmented cells of Rhynchobdellidae never become tubular, and usually remain more or less rounded. In the same group, vacuolated cells and fat cells are found in the connective tissue matrix.
They do not occur in Gnathobdellidae.
The muscles of the body are imbedded in the connective tissue matrix, leaving a sub-hypodermic layer free. They are arranged in an outer circular and a deep longitudinal series. Between the two are diagonal fibres. In Hirudo connective tissue with longitudinal fibres intervenes between the circular and diagonal layers. A set of dorso-ventral and radial fibres takes the place of the septa of Chaetopoda. The fibres of this series near the centre of the body pass between the caeca of the first portion of the alimentary canal. The outer ends of the radial fibres are branched, and end close under the hypodermis in the sub-hypodermic connective tissue. The muscle fibres themselves possess an outer fibrillate layer and an inner granular medulla with a nucleus. Their ends are often much branched, especially on the walls of the alimentary canal.
The amount of connective tissue matrix present, as compared with muscular fibres, is very variable in Hirudinea. Where there is little of it, as in Clepsine and Nephelis, the worm is perfectly firm and rigid to the touch when living. In Aulostoma and Haemopis, where it is plentiful, the animal is always limp. Hirudo occupies a middle position between these two extremes.
Anatomy of the Hirudinea. A. Gibbs Bourne, Q. J. M. xxiv. 1884. Remy Saint-Loup, A. Sc. N. (6) xviii. 1884. Leuckart, Die Parasiten, i. 1863, Leipzig. Moquin-Tandon, Monographic de la famille des Hirudinees, with Atlas (ed. 2), 1846, Paris. Leeches of Japan, Whitman, Q. J. M. xxvi. 1886. Leech, Mcintosh, Encyclopaedia Brit. (ed. ix.), xiv. Genera, etc. Diesing, SB. Akad. Wien, xxxiii. 1858; Id. Systema Helminthum, 1850, i. p. 433, and Leuckart, op. cit.
Somites, Whitman, op. cit. supra; Proc. Amer. Academy, xx. (n.s. xii.) 1885, or American Naturalist, xviii. 1884.
Cuticle, hypodermis, vasifactive and botryoidal tissue. Ray Lankester, Q. J. M. xx. 1880. Dark-green pigmented network of Hirudo. Joseph, Z. A. vi. 1883. Muscle. Shore, Nature, xxvi. 1882, p. 493. Nerve endings in ditto. Hansen, Archives de Biol. ii. 1881.
Eyes and Segmental papillae. Whitman, Q. J. M. xxvi. 1886. Eyes. Car-riere, Sehorgane der Thiere, Munchen und Leipzig, 1885; Ranke, Z. W. Z. xxv. 1875. Ditto and goblet-bodies. Leydig, Arch. f. Anat. und Physiol. 1861; Id. Tafeln zur Vergleich. Anatomie, Tubingen, 1864, Taf. ii. figs. 5, 7; Taf. iii. figs. 1, 2, 3.
Sense-cells of hypodermis. See Gibbs Bourne, op. cit. p. 434, Pl. 27, fig. 15.
Pigment of skin. MacMunn, Proc. Birmingham Phil. Soc. iii. 1881-3, p. 389.