Leech, a red-blooded, footless, smooth-bodied, abranchiate annelid of the family hiru-dinei, and genus sanguisuga (Sav.) or hirudo (Linn.). The body is soft, retractile, composed of numerous segments, with a sucker at the posterior extremity, serving both to attach and to move the animal. The muscular system is well developed, closely embracing the viscera; the sucker has both circular and radiating fibres. The nervous system consists of a large anterior cerebral ganglion, and a chain of ventral ganglia connected by two contiguous cords; the ganglia are fewer than the segments, the first and last being the largest, the former sending filaments to the lips, and the latter to the sucker; there is also a splanchnic system of small anterior ganglia which send filaments to the parts about the mouth and to the intestinal canal. The sense of touch is particularly developed at the anterior extremity. There are ten eye specks symmetrically arranged upon the neck, each a transparent cylindrical body bulging out under the skin like a cornea, enveloped in a layer of black pigment, receiving a filament from the cephalic ganglion, according to Wagner having a lens and a vitreous body (though this is denied by others), and constituting light-perceiving if not light-refracting organs.

The flattened body tapers toward each end, the mouth being at the anterior extremity and provided with a sucking apparatus; at the base of the pharynx are three fleshy swellings, the projecting border of which is edged with bicuspid teeth, causing wounds shaped like a three-rayed star. The intestinal canal is straight, but deeply constricted in many places, each such portion sending off short caeca on each side; the anal opening is on the back directly above the posterior sucker. There are salivary glands around the commencement of the intestine, and a glandular hepatic organ- envelops a great part of this canal. The blood contains colorless granulated globules; there is a central contractile vessel, and a circulation and oscillation in longitudinal and lateral vessels. Respiration is effected by means of 17 pairs of internal branchiae or aquiferous canals without ciliated epithelium, opening upon the ventral surface of the body, and surrounded by a network of blood vessels. Reproduction is effected by sexual organs, and the two sexes are united in the same individual, they being true hermaphrodites; the eggs, from 6 to 15, are contained in a cocoon, surrounded by a thick spongy substance said to be ejected from the mouth, deposited near the edge of the water, and hatched by the heat of the sun; the young leave the egg without undergoing any metamorphosis.

The leech inhabits the water principally, and swims with a vertical undulating motion; out of the water it moves by the disks or suckers, fastening itself first by one and then by the other, alternately stretching and contracting the body; it is torpid in winter, hiding in the mud; it can live a long time in sphagnous moss or in moist earth, and can thus be transported for long distances. Leeches live at the expense of other animals, whose blood they suck; they attach themselves to fishes, batachrians, invertebrates, and to mammals and men that venture into the fresh water inhabited by them. Many species are used for medical purposes, of which the most common are the gray and the green leeches of Europe (S. medicinalis and officinalis, Sav.), generally considered varieties of one species; both have six longitudinal ferruginous stripes on the back, the four lateral ones interrupted by black spots; the back varies from blackish to grayish green; the under parts in the first variety are greenish with black spots and edgings, in the second yellowish green without spots; the length varies from 2 to 4in. They formerly inhabited in great numbers the marshes and streams of most countries of Europe; but of late years the demand for medical purposes has exhausted most of the localities in central and southern Europe; the Swedish leeches are now generally considered the best.

There are many American species, of which the hirudo decora (Sav.) is extensively used in the interior of the middle states; the color is deep greenish above with three rows of square spots, the central brownish orange, and the lateral black; the under parts are spotted with black; it varies in length from 3 to 5 in.; it is especially abundant in Pennsylvania, and several hundred thousand are employed annually. - Leeches afford the least painful and in many cases the only practicable means of local depletion, and are precious instruments in the hands of the physician. They will generally bite eagerly, and will draw from a quarter of an ounce to an ounce of blood, according to the vigor and size of the animal and the vascularity of the part to which it is applied; when full they drop off, though they will sometimes continue to draw after their tails are cut off; the application of a little salt will make them drop at any time; bathing the part with warm water will increase the quantity of blood lost.

When gorged with blood, digestion may not be completed for many months; hence it is customary to strip them by drawing the body between the lingers from the tail to the head, the little that remains serving to keep them in good condition for a long time, if they be kept in clean and frequently changed water. Full leeches are liable to disease and to induce it in others, and should be kept by themselves, not to be used until they have regained their activity; as they often change the slimy coat on their skin, they require moss and roots to draw themselves through in order to keep healthy. In the rare cases in which leech bites bleed too long, the flow may be arrested by pressure, alum solution, caustic, or a superficial suture. The application of leeches requires some skill and attention, and is often usefully placed in the hands of special practitioners, both male and female. - The horse leech (haemopis, Sav.) is a larger species, differing principally by the oval and slightly toothed jaws; it will not attack man, and it is doubtful if it attaches itself to horses and other animals; it devours other worms, swallowing them whole. - The leech family is a large one, and can be studied only in special treatises, of which along list is given in the chapter on annelids in Siebold's " Comparative Anatomy."

Leech (Sanguisuga medicinalis).

Leech (Sanguisuga medicinalis).

1. Leech. 2. Anterior extremity magnified. 3. Jaw detached, magnified. 4. Part of belly magnified.