There are as many as twelve or fifteen species of these creatures, but only two have been employed in medicine, namely, the Hirudo medicinalis, which may be known by having six yellowish lines, or striæ, on its back, while the under part is of a grayish hue spotted with black, but, as we shall presently see, these markings are not uniformly found; and the Hirudo troctina, of a brownish colour, the upper part of the body marked with black spots, each of which is surrounded with a golden-coloured ring, the sides of a dingy yellow, and the under part of a yellowish green with black spots.
The first of these species, the medicinal leech, is common throughout the whole of Europe, but is much more abundant in the Southern parts; it is generally about three inches in length. Formerly it was very abundant in Great Britain, but the improvements in agriculture, and the consequent drainage of the land, together with the great use made of it in medicine, have of late years rendered it of less frequent occurrence. On this account great quantities of leeches are imported; these chiefly come from Bourdeaux and Lisbon. On a moderate calculation, it appears that, in England, on an average, out of every hundred leeches employed, ninety-nine may be considered of foreign production; these differ from the English leech in being somewhat larger, and having the under part of a uniform colour, without spots. Some idea may be formed of the number of leeches used in medicine by the statement, that in the hospitals of Paris alone, 300,000 were employed in one year. The prevailing colour of the medicinal leech appears to vary according to the nature of the soil on which it is found. In Winter the leech retires to waters of considerable depth, and seeks shelter in the mud at the bottom; but in the Summer it appears to delight in shallow pools, basking, as it were, in the warmth of the sun: but if the water it frequents is in danger of being dried up by the Summer-heat, the leech buries itself in the mud at a considerable depth. Just before a thunder-storm, leeches appear much agitated, and rise frequently to the surface of the water; this, therefore, is considered by the leech-gatherers as a favourable time for collecting them.
The property by which a leech anticipates thunder, has induced some persons to employ it as a species of barometer; for this purpose a leech is enclosed in a glass vessel half-filled with water, and the following is supposed to be the result. When the weather is about to be serene and pleasant, the leech will remain at the bottom of the vessel without the least movement; secondly, if it is about to rain, the animal will rise to the surface, and there remain until the approach of fine weather; thirdly, before boisterous weather, it will appear in a state of great agitation; fourthly, on the approach of thunder, it will remain out of water for several days, appearing agitated and restless, and so on.
This natural barometer appears to answer tolerably well, if there is sufficient belief in its virtues on the part of the possessor, and if one leech only is employed; but when several of these creatures are enclosed in the same vessel, they do not appear to obey the same laws, and, consequently, their movements do not correspond with sufficient accuracy to render their indications of the weather of much use.
The medicinal leech appears during its whole life to exist on the blood or other juices of the creatures on whose body it fixes itself; this is not the case with the horse-leech, which lives entirely on the larvæ of aquatic insects, worms, &c., so that the common idea of the danger of the bite of the horse-leech is without foundation.
The horse-leech is exceedingly voracious, not only swallowing worms, tadpoles, &c., but even preying upon its own species. Sixty-five horse-leeches were placed in a glass vessel, and in five days the number was reduced to fifty-two, and not a vestige of those that were missing was to be discovered.
The usual slowness of action of the digestive powers in all animals of cold blood, was curiously illustrated in the case of a horse-leech, which, after swallowing two small leeches of a different species, disgorged one of the two at the end of three days, in a living state, and apparently not much injured from its sojourn in so unusual a lodging; but it enjoyed its liberty only for a few hours, its more powerful companion swallowing it a second time at the end of that period.
A number of this species of leech, inhabiting the water that supplied a trough in which a tench had been placed, fixed themselves to different parts of the body of the fish, and so effectually was the poor tench annoyed, that it was soon deprived of life. “The leeches then tore it (previously breaking the line of connexion between the various parts of the body, by inflicting a vast number of bites or wounds,) into such pieces as they could readily receive into the stomach, and so diligent were they, that in a few days nothing remained of the fish but the mere skeleton.”
From these habits it would appear, that the name of Hirudo sanguisuga, (the blood-sucking leech,) has been improperly applied to the horse-leech; on this account a recent author has suggested the name of Hirudo vorax, (the voracious leech,) as being more suitable to its nature.
Leeches are supposed to be very long-lived; two were preserved in confinement for eight years before they died, and the well-ascertained slowness of their growth seems to place their length of life beyond a doubt.