The American Cyclopædia (1879)/Leech - Wikisource, the free online library (2024)

LEECH, a red-blooded, footless, smooth-bodied,abranchiate annelid of the familyhirudinei, and genus sanguisuga (Sav.) or hirudo(Linn.). The body is soft, retractile, composedof numerous segments, with a sucker at theposterior extremity, serving both to attach andto move the animal. The muscular system iswell developed, closely embracing the viscera;the sucker has both circular and radiating fibres.The nervous system consists of a large anteriorcerebral ganglion, and a chain of ventralganglia connected by two contiguous cords; theganglia are fewer than the segments, the firstand last being the largest, the former sendingfilaments to the lips, and the latter to the sucker;there is also a splanchnic system of smallanterior ganglia which send filaments to theparts about the mouth and to the intestinalcanal. The sense of touch is particularly developedat the anterior extremity. There are teneye specks symmetrically arranged upon theneck, each a transparent cylindrical body bulgingout under the skin like a cornea, envelopedin a layer of black pigment, receiving a filamentfrom the cephalic ganglion, according toWagner having a lens and a vitreous body (thoughthis is denied by others), and constitutinglight-perceiving if not light-refracting organs.The flattened body tapers toward each end, themouth being at the anterior extremity andprovided with a sucking apparatus; at the baseof the pharynx are three fleshy swellings, theprojecting border of which is edged with bicuspidteeth, causing wounds shaped like a three-rayedstar. The intestinal canal is straight, butdeeply constricted in many places, each suchportion sending off short cæca on each side;the anal opening is on the back directly abovethe posterior sucker. There are salivary glandsaround the commencement of the intestine,and a glandular hepatic organ envelops a greatpart of this canal. The blood contains colorlessgranulated globules; there is a centralcontractile vessel, and a circulation and oscillationin longitudinal and lateral vessels. Respirationis effected by means of 17 pairs of internalbranchiæ or aquiferous canals without ciliatedepithelium, opening upon the ventral surfaceof the body, and surrounded by a network ofblood vessels. Reproduction is effected by sexualorgans, and the two sexes are united in thesame individual, they being true hermaphrodites;the eggs, from 6 to 15, are contained ina cocoon, surrounded by a thick spongysubstance said to be ejected from the mouth, deposited near the edge of the water, and hatchedby the heat of the sun; the young leave theegg without undergoing any metamorphosis.The leech inhabits the water principally, andswims with a vertical undulating motion; outof the water it moves by the disks or suckers,fastening itself first by one and then by theother, alternately stretching and contractingthe body; it is torpid in winter, hiding in themud; it can live a long time in sphagnous mossor in moist earth, and can thus be transportedfor long distances. Leeches live at the expenseof other animals, whose blood they suck; theyattach themselves to fishes, batrachians,invertebrates, and to mammals and men that ventureinto the fresh water inhabited by them. Manyspecies are used for medical purposes, of whichthe most common are the gray and the greenleeches of Europe (S. medicinalis and officinalis,Sav.), generally considered varieties ofone species; both have six longitudinal ferruginousstripes on the back, the four lateralones interrupted by black spots; the back variesfrom blackish to grayish green; the under partsin the first variety are greenish with black spotsand edgings, in the second yellowish greenwithout spots; the length varies from 2 to 4 in.They formerly inhabited in great numbers themarshes and streams of most countries ofEurope; but of late years the demand for medicalpurposes has exhausted most of the localitiesin central and southern Europe; the Swedishleeches are now generally considered the best.There are many American species, of whichthe hirudo decora (Sav.) is extensively used inthe interior of the middle states; the color isdeep greenish above with three rows of squarespots, the central brownish orange, and thelateral black; the under parts are spotted withblack; it varies in length from 3 to 5 in.; itis especially abundant in Pennsylvania, andseveral hundred thousand are employed annually.—Leeches afford the least painful and inmany cases the only practicable means of localdepletion, and are precious instruments in thehands of the physician. They will generallybite eagerly, and will draw from a quarter ofan ounce to an ounce of blood, according to thevigor and size of the animal and the vascularityof the part to which it is applied; when fullthey drop off, though they will sometimescontinue to draw after their tails are cut off; theapplication of a little salt will make them dropat any time; bathing the part with warm waterwill increase the quantity of blood lost. Whengorged with blood, digestion may not becompleted for many months; hence it is customaryto strip them by drawing the body betweenthe fingers from the tail to the head, the littlethat remains serving to keep them in goodcondition for a long time, if they be kept in cleanand frequently changed water. Full leechesare liable to disease and to induce it in others,and should be kept by themselves, not to beused until they have regained their activity;as they often change the slimy coat on theirskin, they require moss and roots to drawthemselves through in order to keep healthy.In the rare cases in which leech bites bleed toolong, the flow may be arrested by pressure,alum solution, caustic, or a superficial suture.The application of leeches requires some skilland attention, and is often usefully placed inthe hands of special practitioners, both maleand female.—The horseleech (hæmopis, Sav.) isa larger species, differingprincipally by theoval and slightly toothedjaws; it will not attackman, and it is doubtfulif it attaches itself tohorses and other animals;it devours otherworms, swallowingthem whole.—The leechfamily is a large one,and can be studied onlyin special treatises, ofwhich a long list is givenin the chapter on annelidsin Siebold's“Comparative Anatomy.”