LEECH 403
The mesoblast now forms two lateral curved bands at the sides of the body. The three vitelline spheres become covered with the flattened cells of the epiblast. The cephalic region becomes ciliated, and the cilia enter the œsophagus. The epiblast develops the cuticle, which is raised into transverse rings, without, however, having any relation to the true segments of the mesoblast. The nervous system is probably derived from the epiblast, the ventral cord breaking up into a series of ganglia, which correspond with the true somites, except that the first and last, as already mentioned, are composed of several. The supra-cesophagal ganglia arise independently. The mesoblast probably takes its origin from the two mesoblastic bands, and the segments formed by it grow upward and meet in the dorsal line, and septa are formed between the somites. The somatic layer of the mesoblast gives rise to the muscles. The mesoblast also gives origin to the excretory (segmental) and generative organs, and the vascular system. A delicate musculature, however, would appear to be developed independently of the mesoblastic bands. The mouth and pharynx are formed by the epiblast, the rest of the canal by the hypoblast, which from the first has a sac-like shape. The posterior sacculation of the stomach in Hirudo is originally unpaired. The dental pads are formed about the same time as the eyes as protuberances of the oral cavity. The anus is developed very late above the posterior sucker. In the embryo of Hirudo Leuckart found three pairs of segmental organs at the posterior end of the body, consisting of an enlargement from which a convoluted tube is continued for some distance backward, and then bends forward to open on the exterior. The anterior part is broken up into a labyrinthic network. These organs disappear in the adult. The recent researches of Whitman on Clepsine and of Hoffmann have greatly extended our information with regard to the histology and morphology of the parts in the embryos of the leeches.
The time between the deposition of the ova and their hatching is variable, and probably depends, as in the ova of the Salmonidæ, on temperature and other causes. It is said to range from twenty-five to forty days. The young arrive at perfect coloration when two years old, and become sexually mature at three years, about which age they become fit for medicinal use; their food consists at first of microscopic organisms, and afterwards, when the mouth has attained more complete development, of the larvæ of insects and other small animals.
There is no annelid that has been more prominently brought under notice than the leech, both on account of its use in medicine from very early times, and its fitness for anatomical and other investigations. The number of treatises, inaugural, historical, and structural, that have been devoted to it is very considerable; of these the voluminous article in Brandt and Katzeburg's Medicinische Zoologie may be taken as a type.
Medicinal use.
The leech, is the (
Greek characters) of Herodotus, Theocritus, Nicander, and other Greek authors, and the Hirudo and Sanguisuga of Plautus, Cicero, Horace, Pliny, and other Roman writers. Cælius Aurelianus mentions its use, and Galen and his successors recommend its application. Appian also alludes to the latter, and describes very graphically the process by which it fills itself with blood. It was sufficiently familiar to naturalists both before and after the time of Linnæus, though occasionally there has been considerable ambiguity in regard to species. The use of the leech is mainly for local blood-letting, but in modern times the practice has greatly diminished; indeed, in some cities the druggists chiefly use them with doubtful efficiency in cases of incipient gumboil and in facial ecchymosis. They may be applied to any part of the adult skin, and to the mouth, fauces, and other available inlets by the aid of a leech-glass, which consists of a tube with a slightly contracted aperture, and provided (or not) with a glass piston to push the leech onward. In China a piece of bamboo serves the same purpose. For such functions the most active specimens should be chosen (and, as Sir Robert Christison states, these contract firmly when squeezed in the hand) and kept for an hour out of water, and then applied to a perfectly clean surface of skin. They may also be made to bite by smearing the skin with cream or blood, or by immersing the leech for a minute in porter or tepid water. Each fills in about fifteen minutes, and draws from 40 to 85 grains of blood, or, including that afterwards obtained by fomenting the wound, about half an ounce. In young children they should never be placed on parts where firm pressure cannot be applied. It was formerly the practice to prepare the leeches that had been used for further action by sprinkling a few grains of salt on the snout, and stripping them gently between the fingers so as to cause them to eject the blood. This plan is not now adopted, and rightly so, since various diseases might thus be communicated. They certainly can be applied four and five times in succession by placing them in vinegar and water, and afterwards in a vessel (which the French call a domestic marsh) with turfy earth; but they draw less blood on the fifth occasion. Should the hæmorrhage from the wounds (as in certain constitutions) prove severe, it may be staunched by the application of vinegar, solid nitrate of silver, a hot wire, or a hot solution of alum, or by acupuncture. If a leech by accident be swallowed, a pretty strong solution of common salt, or a glassful or two of wine may be taken. Instead of the actual leech an instrument called an artificial leech is now sometimes used. This consists of a small sharp steel cylinder (worked by a spring) with which a circular incision can be made through the skin, and a glass cylinder capable of being exhausted by a piston worked by a screw. Care must be taken to move the piston at about the same rate as the blood flows, and the edge of the glass cylinder should not press too tightly, else the flow is arrested.
Leeches are imported from France and Hungary, and also through Hamburg from Poland and the Ukraine; they likewise come from Turkey, Wallachia, Russia, Egypt, and Algeria. They are found in Britain – both in Scotland and England, but especially in the latter. In the French trade Bordeaux leeches are preferred; Polish, Swedish, and Hungarian are those most commonly met with in Britain. It is difficult to estimate the number of leeches now used. In 1846 Moquin-Tandon calculated that there were from twenty to thirty millions used in France; and Leuckart mentions in 1863 that in London seven millions, and in the Parisian hospitals five to six millions, were annually employed At the great American leech-farm the average sale is one thousand per day. There cannot be a doubt, however, that the use of leeches at the present time is greatly restricted indeed, the younger generation of British medical men seldom or never prescribe them – so that scarcely one will now be employed where one hundred were a quarter of a century ago. This is very well shown in a note from Messrs Duncan, Flockhart, & Co. of Edinburgh, from which it appears that the account for leeches supplied during three months in 1844 to the Royal Infirmary, Edinburgh, was £45. This steadily decreased until about 1868 it amounted for the same period only to 5s. 6d. Sir Robert Christison mentions that the price of the best leeches in 1845 ranged from £4 to £8 per thousand; twenty years ago they were from £10 to £15 per thousand; and at the present time good leeches cost about 10s. per hundred, or £5 per thousand.
They inhabit ditches and ponds, with pure running water, weeds for shelter, and muddy banks and bottom. They are captured by nets after attracting them by baits, or by wading into the water, and then stripping them off the legs on coming to land. Leeches are preserved in loose turf or moss constantly moistened, or in earthenware or glass vessels half full of water, covered with glass or linen-gauze; and some place a rusty nail, others a clean sponge in the vessel, which can be exposed to the light. In transporting them the French "domestic marsh," a vessel with small perforations inferiorly and filled with moist turfy earth or peat made into a stiff mud, is excellent. Sometimes an exterior vessel with a few inches of water is placed round the former. The mouth of the vessel is closed with a coarse linen cloth. Leeches, like many other annelids, live for several years without food in vessels of pure water.
