Part I

THE MEDICINAL LEECH (Hirudo medicinalis)

Hardly anything real in the shop but the leeches and they’re

second-hand. (Bob Sawyer, The Pickwick Papers.)

As Mr. W. A. Harding has pointed out, eleven species of fresh-water leeches occur in these islands. But one of these, the Hirudo medicinalis, seems to be vanishing, and yet it is just the one we should cherish and preserve.

Fig. 48.—Hirudo medicinalis; about life size. 1, Mouth; 2, posterior sucker; 3, sensory papillae on the anterior annulus of each segment. The remaining four annuli which make up each true segment are indicated by the mark­ings on the dorsal surface.

There are people who do not like leeches. This is shown by the agitation amongst the travellers in an omnibus, as depicted in Punch by Leech, years and years ago, when an old gentleman had upset a bottle of them in their midst. But the medicinal leech, which is our theme, is really the friend of man and of the soldier, and is a beneficial and not a harmful animal. There are, of course, other leeches in our rivers and in our seas, but of the latter our knowledge is scanty and it is difficult to increase it at present—at any rate, in the Channel or in the North Sea. In any case the marine leeches in our island-waters have no human interest except the influence they exercise on our fish-food supply, and this is practically negligible.

Zoologically speaking, leeches are undoubtedly degenerate earth worms (Oligochaeta); and some very interesting ‘Zwischenformen’—like Mr. Vincent Crummles, I am ‘not a Prussian’; but in spite of the war, we may as well employ a useful term captured from the enemy—have been found in Russia and Siberia: forms which combine many of the characters of the Oligochaeta and the Hirudinea. Possibly the degeneracy which leeches are said to exhibit is associated with a semi-parasitic habit of life. But a semi-parasitic habit does not apply to all leeches—in fact, it applies but to few genera; there are many others, equally degenerate—if degenerate they be—who have no trace of semi-parasitism.

A curious thing about leeches is that all the varying genera have the same number of somites or segments; and though some of these segments or somites are masked and fused, when analysed by the number of segments in the embryo, by the number of the nerve ganglia, and so on, leeches seem always to have thirty-four such segments. These do not correspond with the rings or annulations so visible on the outside; but a certain number of these annulations, varying in each species, ‘go’ to each somite, and so constant are these numbers that it would not be very difficult to represent any given species of leech by a mathematical formula.