The known species readily fall into two sub-orders: (1) The Rhynchobdellae, which are marine and fresh-water leeches with colourless blood, with no jaws, and with an extensile proboscis; and (2) the Arhynchobdellae, which are all fresh-water or terrestrial, with red blood, and generally with jaws. There is no extensile proboscis, and the anterior sucker has a ventral aspect, and is in no way distinct from the body. There are always in this group seventeen pairs of nephridia or kidneys. We shall have mostly to do with the latter sub-order.
Fig. 49.—View of the internal organs of Hirudo medicinalis. On the left side the alimentary canal is shown, but the right half of this organ has been removed to show the excretory and reproductive organs. 1, Head, with eye-spots; 2, muscular pharynx; 3, first diverticulum of the crop; 4, eleventh diverticulum of the crop; 5, stomach; 6, rectum; 7, anus; 8, cerebral ganglia; 9, ventral nerve-cord; 10, nephridium; 11, lateral blood-vessel; 12, testis; 13, vas deferens; 14, prostate; 15, penis; 16, ovary; 17, uterus, a dilatation formed by the conjoined oviducts.
Hirudo medicinalis, the medicinal leech, is found in stagnant waters throughout Europe and the western parts of Asia. It is rather commoner in the southern parts of Europe than in the north. It used to be common enough in England, where at one time, it was bred; but already a hundred years ago its numbers were diminishing.
In a treatise on the Medicinal Leech, published by J. R. Johnson in the year 1816, he records: ‘Formerly the species was very abundant in our island; but from their present scarcity, owing to their being more in request among medical men, and to the rapid improvements which have of late years taken place in agriculture—particularly in the draining and cultivation of waste lands—we are obliged to receive a supply from the Continent, chiefly from Bordeaux and Lisbon.’ In his time he considered that for every native leech employed at least a hundred foreigners were used.
The same scarcity was very apparent to the poet Wordsworth, whose insatiate curiosity is recorded in the following lines in 1802—Wordsworth was always asking rather fatuous questions:—
My question eagerly did I renew,
‘How is it that you live, and what is it you do?’
He with a smile did then his words repeat:
And said that, gathering leeches, far and wide