wards, as in the act of biting with great force and rapidity; one muscle appears to be continuous with that of the oesophagus, and is attached to one extremity of the jaw, whilst another is attached to the opposite end, so that by their alternate contraction and dilatation the jaws have a semicircular movement.
But the dental apparatus of the medicinal leech is much more marked than in this species, and this fact, combined with the striking differences in their alimentary canals, affords a most satisfactory explanation of the reason why the horse leech is unfit for medical purposes, for instead of the fifteen blunt molar-like teeth with which the latter is provided, the jaws of the medicinal species are much larger, more convex, and of very much firmer consistence; each jaw is provided with a row of teeth, varying from sixty to eighty in number; their points or apices are much sharper than those of the finest needles (fig. g); they are likewise strongly imbedded in the cartilaginous matter of the jaw: in the horse leech, on the contrary, the teeth appear to be loosely connected to the upper part of the jaw, for when a leech has undergone slight decomposition the little calcareous teeth readily separate from it, and, on examining the mouth and squeezing it, the three sets of teeth will be found quite detached, being only entangled in the mucous matter, which is always very abundant at this part; they can then be washed in water and placed on glass, and mounted as microscopic objects.
I cannot forbear mentioning, in this place, an error which is likely to be committed in describing the shape of the teeth: if the jaws of the medicinal leech be examined, when laid on their sides, the teeth appear like so many sharp-pointed canines, imbedded, like the teeth of Mammalia, to half their depth in the cartilage of the jaw, all that part projecting above the margin of the jaw being of a darker colour than the remaining half which appears imbedded in it: that this is deceptive may be at once proved by turning the jaw in such a manner that the cutting edge may be seen (fig. h); the teeth then will be found to be of precisely the same figure as those of the horse leech, (fig. f), the only differences being that they are more numerous and much sharper than in the latter animal. Having fallen into this error myself, and only by mere accident discovered the mistake, I have thought proper to caution others: Moquin Tandon has represented two rows of teeth in each jaw of the medicinal leech, and makes each tooth pointed like a canine; in this he is certainly wrong.
(To be continued).
