LEECH, the popular name of members of the Hirudinea, a branch of the Chaetopod worms. In Anglo-Saxon the word leech or laece was used both for the blood-letting medicinal Hirudo and for the physician, but it is uncertain whether the animal was named after the physician or the physician after the animal. From their association with man, both as pests and as surgical agents, the leeches have acquired considerable importance. In tropical countries the land leeches are unanimously regarded as the most noxious of external parasites, and there are many ac counts of persons being seriously injured or killed by their attacks.
Haeckel writes of the leeches of Ceylon : "In some of the for ests, and particularly near the river banks, and the marshy jungles of the highlands and lower hills, it is impossible to take a step without being attacked by them. Not only do they creep along the ground seeking what they may devour—they are on every bush and tree, from which they frequently drop on the head and neck of the passer-by : while they always creep up his legs, nay, they can even spring to meet their victim." This ac count is borne out by all who are acquainted with the conditions in tropical forests.
In Egypt and the Near East an aquatic leech Limnatis nilotica, which lives in streams and ponds, is a serious menace to men and animals. It is swallowed with drinking water, and fastens itself to the larynx, the epiglottis, and the nasal cavities of the host. The result is a constant haemorrhage, which, if the parasite is not removed, may prove fatal. Napoleon's army in Egypt suffered from this leech, as did the soldiers in Palestine during the World War.
Although up to the present there has been no comprehensive record of the extent of damage done by leeches to domestic ani mals, the well-authenticated accounts by travellers and others of horses and baggage animals being severely injured by them, justify the assumption that this damage is considerable. Accord ing to one authority the drain on the American fisheries from these creatures is appreciable; and there is some evidence that in the Philippines the common rice-field leech is the carrier of the infective organism of rinderpest. J. P. Moore suggests that as this and closely related species are widely spread throughout.
India and Ceylon, they may be suspected of aiding in the trans mission of this virulent cattle disease. Although a leech-bite is not specifically toxic, their habit of fastening on to ulcers and other diseased areas of their host's body constitutes them perfect carriers of bacterial infections.
As surgical agents they have largely fallen out of use in Europe. The late Sir Arthur Shipley showed that in India and Arabia leeches have been used in phlebotomy since the beginning of the Christian era ; there are frequent references to the practice in Europe to be found in Pliny's Natural History. This mode of blood-letting reached its zenith in Europe during the early 19th century. Ebrard records that 574 million of these creatures were imported into France in the year 1832, leech-farming being at that time a profitable industry. Huge quantities were brought in from Russia, Bessarabia, Astrachan and Trans-Caucasia, where the sectarian exiles made money by their exportation. The Rus sian Government actually passed a special "game-law" in 1848 forbidding their collection in May, June and July. Wordsworth wrote a fine poem entitled "The Leech-Gatherer." Although leeches no longer form an essential part of the equip ment of the modern surgeon, they still serve the cause of science. Their saliva contains a haemolysin known as hirudin, which, when mixed with the blood of a wound, prevents coagulation. This substance is used in surgery and was much in demand in the World War, when India, where practitioners of Ayurvedic , medicine still freely use leeches for blood-letting, became the main source of supply.
Another purpose for which leeches have been used is that of a barometer. Blainville records that in certain parts of France the country folk keep leeches in a bowl of water with earth at the bottom, and foretell the weather by the degree of their elevation in the bowl. Belief in their efficacy was not confined to the un educated, for Dr. J. Forster stated 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." This view, however, is not shared by the meteorolo gist of to-day. See ANNELIDA : Hirudinea. (C. C. A. M.)