a body destitute of bristles for locomotion, completely apodous, and without soft appendages, a prehensile cavity, in the form of a sucker, at each extremity of the body. Head not distinct, but generally provided with eyes and jaws. External form. The common horse leech varies but little in size; it rarely exceeds three inches in length in its contracted state: the general colour is dark green, sometimes inclining to black on its dorsal, and green or greenish yellow on its ventral aspect; on some specimens a few irregular black dots occur in various parts, and in others the dots are so numerous, and approach each other in a linear series so closely on the back of the animal, as to give the appearance of two black lines or bands, running from one extremity of the body to the other.
The body itself is simple, elongated, tapering slightly towards the extremities, both of which are provided with a dilatable cavity or sucker, which is prehensile, that on the head being termed the anterior or oral sucking disc, and that on the tail the posterior or anal. The dorsal surface, as in the Hirudinidæ generally, is convex, whilst the ventral is flattened in the middle and convex at the margins. The surface of the body is soft and smooth when the animal is extended, but when in its contracted state it is rugose, from a number of papillae which appear and disappear at the will of the animal; it is composed of a series of rings, which are said to increase in number as the growth advances, but such is not the case; as many rings may be counted on a young individual as on one of full size; the number varies from ninety to a hundred in the majority of specimens of this species.
On a careful examination of the ventral surface, certain pores or openings may be detected: in the middle of about the twentieth ring, reckoning from the mouth downwards, the male genital pore occurs, and five rings below this is the opening of the female genital organs, as shown in fig. a; these are readily made out by the unassisted eye, as they are always tolerably conspicuous; but, by means of a pocket lens, a number of equal-sized apertures, called spiracles or stigmata, may be seen, arranged in two rows, one on each side of the body; they occur at every fifth ring, and communicate with some little sacs or cavities, which are of a white colour, and in general are filled with an opaque white fluid when the animal has been dead some little time, but in a living leech the fluid appears as transparent as water: their supposed use will be adverted to in a subsequent part of this communication.
