The Minor Horrors of War

by Sir A. E. Shipley

The shape of the medicinal leech, and indeed of other leeches, is difficult to put into figures, as their bodies are as extensile as the conscience of a politician and as flexible as that of a candidate for parliamentary honours. The length of H. medicinalis in extreme extension is said to range from some 100 mm. to 125 mm.; in extreme constriction from 30 mm. to 35 mm. The width in the former state would be 8 mm. to 10 mm., and in the latter 15 mm. to 18 mm.

The movements of the medicinal leech are as graceful as its colour is tasteful. When in the water they move like looper-caterpillars (Geometrids), stretching out their anterior sucker, attaching it to some object, and then releasing the posterior sucker they draw the body up towards the mouth. Or, casting loose from all attachment, the leech elongates and at the same time flattens its body until it assumes the shape of a band or short piece of red tape, and by a series of the most seductive undulations swims through the water. Kept in an aquarium they are rather apt at times to leave the water and take up a position on the sides of their home an inch or two above the aqueous surface. When outside the water they keep their bodies moist by the excretion of their nephridia or kidneys. This fluid plays the same part on the skin of a leech as the coelomic fluid of an earthworm, which escapes by the earthworm’s dorsal pores. There is very little doubt that both these fluids contain some bactericidal toxin which prevents epizootic protozoa and bacteria from settling on their skins. Such external parasites settle on many fresh-water crustacea—such as Cyclops, which is a floating aquarium of Ciliata. In fact, leeches, like earthworms, have a self-respecting, well-groomed external appearance. Like our dear soldiers, they are, so to speak, always clean shaven.

There has been a very widely spread tradition that in their comings and goings in and out of the water, leeches act as weather prophets. The poet Cowper, who throughout his chequered career ever showed but an imperfect sympathy with science, tells us that ‘leeches in point of the earliest intelligences are worth all the barometers in the world’; and Dr. J. Foster mentions that leeches, ‘confined in a glass of water, by their motions foretell rain and wind, before which they seem much agitated, particularly before thunder and lightning.’ Modern opinion, however, prefers the barometer.

The great Chancellor, Lord Erskine, kept a couple of tame leeches and Sir Samuel Romilly records the fact in one of his decorous letters:—

He told us how that he had got two favourite leeches. He had been blooded by them last autumn when he had been taken dangerously ill at Portsmouth; they had saved his life, and he had brought them with him to town, had ever since kept them in a glass, had himself every day given them fresh water, and had formed a friendship with them. He said he was sure they both knew him, and were grateful to him. He had given them different names, Home and Cline (the names of two celebrated surgeons), their dispositions being quite different. After a good deal of conversation about them, he went himself, brought them out of his library, and placed them in their glass upon the table. It is impossible, however, without the vivacity, the tones, the details, and the gestures of Lord Erskine, to give an adequate idea of this singular scene. He would produce his leeches at consultation under the name of ‘bottle conjurers,’ and argue the result of the cause according to the manner in which they swam or crawled.[15]

The medicinal leech lives on the blood of vertebrates and invertebrates. Mr. H. O. Latter records that ‘cattle, birds, frogs and tadpoles, snails, insects, small soft-bodied crustacea, and worms are all attacked by various species’ of leech; but the true food of Hirudo medicinalis is the blood of vertebrates. The three teeth, which cause the well-known triradiate mark on the skin, are serrated and sharp. The strong sucking-pharynx has its wall attached by numerous muscles to the underside of the skin of the leech. By the contraction of these muscles its lumen is enlarged, and by thus creating a vacuum the blood of the host flows in.

In the walls of the pharynx and the neighbouring parts are numerous large unicellular glands which secrete an anti-coaguline fluid which prevents the blood of the host clotting, so that even when the leech moves its mouth to another point the triradiate puncture continues to ooze. The same anti-coaguline secretion no doubt prevents the blood coagulating in the enormous crop of the leech in which this meal of blood is stored. Opportunities for a meal presumably occur but seldom in nature, and the leech is the ‘boa-constrictor’ of the invertebrate world. Its interior economy is laid out on the basis of a large and capacious storage and of a very restricted and very slow digestion. The blood sucked into the sucking-pharynx passes on to the thin-walled crop, which occupies almost all of the space in the animal. This crop is sacculated, having eleven large lateral diverticula on each side. In a fed leech the whole of this crop is swollen with blood, which, as we have said above, does not coagulate. The actual area where the digestion takes place is ludicrously small, as shown at 5, [Fig. 49, p. 126]. The rectum, which runs from the real seat of assimilation to the opening of the posterior sucker, transmits the undigested food—but there is not much of it.

An active medicinal leech will draw from one to two drams of blood, and as much more will flow from the wound when the leech moves, because the coagulation of the blood has been put out of action. No scab or clot is formed. If necessary, the flow of blood can be stimulated by hot fomentations. Sometimes the bleeding is so great that artificial means have to be taken to check it. When leeches are applied to the human integument they are generally first dried in a cloth, and if they will not bite the part required, the part should be moistened with sweetened milk or a drop of blood. To remove leeches when replete, salt, sugar, or snuff sprinkled over the back is used. They may then be made to disgorge by placing them in a salt solution of 16 parts salt and 100 of water at 100° F. A full meal is said to last leeches nine months.

Fig. 52.—Cocoon of the medicinal leech, and longitudinal and transverse views of the same cut open.