On the head of the medicinal leech ten points are arranged in the form of a horse-shoe, thus—
These are considered, by some authors, to be organs of sight, or eyes, while, on the other hand, it is stated by others, that they are merely tubercles. Lamarck was of this opinion, and, consequently, in describing their character, says they are without eyes.
The teeth, or rather piercers, with which the leech is furnished, are three in number, of a hard gristly substance, and so placed, with regard to each other, as to meet in the centre at equal angles; these piercers are thrust into the skin when the animal attaches itself; not by one plunging effort, but by constantly scratching or sawing upon the surface (assisted at the same time by the sucking action of the lips); in this manner they gradually become buried in the skin, and there remain as long as the creature retains its hold; this movement of the piercers occasions the gnawing pain felt for the first two or three minutes after the leech has commenced operation.
Leeches are at times so scarce and valuable, that great care has been taken in preserving them in a healthy state and fit for use. The principal art in managing them consists in placing them in vessels sufficiently large, keeping the water clear, and in removing those which are unhealthy as soon as they are discovered.
Leeches, when applied to the skin, frequently show little inclination to bite, and many plans have been resorted to, to induce them to commence operations, such as bathing the part with milk, &c.; but these methods may be considered useless, and the best plan appears to be, to wash the part clean, and this is the more necessary when any embrocation has been previously applied; but the surest way is to puncture the place slightly, so as to cause the blood to appear. If the little surgeon, before it is fully gorged, appears lazy and unwilling to proceed, it can be usually roused by being sprinkled with a little cold water.
After a leech has fallen off, it is usual to sprinkle salt on it to induce it to disgorge the blood it has swallowed; but as the salt frequently blisters its body, it has been recommended by Dr. Johnson of Edinburgh, from whose work on the Leech, we have obtained most of the preceding information, to apply a small portion of vinegar to the head of the leech instead of salt.
The necessity for obliging the leech to dislodge the blood it has swallowed, arises from the fact that it would remain in the body of the animal for some months before it could be all digested; but the most singular thing is, that, during the whole of this time, the blood remains in nearly as fluid a state as when it was newly swallowed[5]. The stomach of this creature is very curiously formed, being composed of a number of chambers, each chamber having a separate connexion with the intestinal canal, in such a manner that, at the will of the animal, the contents of each chamber can be emptied singly into that canal, through a distinct opening.
Fig. 1. Fig. 2.