Suspended to show the external form of the body and the coloured bands which differentiate the variety H. medicinalis from the variety //. officinalis.
THE animal is suspended by the anterior extremity which is formed by the funnel-shaped buccal cavity or anterior sucker which leads to the mouth, and is not separated from the rest of the body by a constriction in Leeches with jaws (Gnathobdellidae) as it is in Leeches with a protrusible proboscis (Rhynchobdellidae). The body itself has a flat or slightly concave ventral surface and a convex dorsal surface: and it is terminated by a disclike solid posterior sucker which is formed by the fusion of posterior embryonic somites, according to Leuckart seven in number.
The body is annulated, and, according to Whitman, H. medicinalis has in all one hundred and two rings, representing twenty-six somites. Consequently the annuli do not represent somites: they are, on the contrary, due to a secondary and imperfect division of them. It has been pointed out by Whitman that certain of the annuli or rings bear what he terms segmental papillae. These organs resemble in histological structure the eyes, but reduced in size and deprived of pigment, and they may be regarded as the metameric or serial homologues of those organs. In Hirndo and some other Leeches there are normally fourteen segmental papillae, eight on the dorsal and six on the ventral aspects of the annuli upon which they occur. The eight dorsal papillae are arranged as a median pair, with three organs to either side of it, an inner, outer, and marginal organ. The first pair of eyes replaces the median pair of papillae, the remaining four pairs a papilla of the inner series. The first and second annuli of the body carry a pair of eyes; the third annulus does the same, but it is followed by an annulus with neither eyes nor segmental papillae: and the two therefore go together.
The fifth annulus bears eyes; the eighth and eleventh only segmental papillae, and these three annuli (fifth, eighth, and eleventh) are each followed by two annuli which have neither eyes nor papillae. Whitman therefore concludes that the first and second somites are represented by a single annulus, the third by two annuli, the fourth to the sixth inclusive by three apiece. The seventh and succeeding somites, up to the twenty-second inclusive, are each composed of five annuli, the first of them bearing the segmental papillae. The twenty-third somite has three, the twenty-fourth to the twenty-sixth two annuli apiece.
The buccal annuli are the fifth and sixth, the post-buccals the seventh and eighth, and these two pairs of annuli are fused ventrally. A pair of nephridial pores opens on the ventral surface of the last annulus of the sixth to the twenty-second somite inclusive. This annulus can be readily recognised in H. medicinalis by the fact that it is the one that carries a large black spot in the middle of the three light lines which traverse the body lengthwise on each side of its dorsal aspect. The male orifice lies between the second and third rings of the tenth somite, i. e. the thirtieth and thirty-first annuli of the body. The female orifice occupies a corresponding position in the eleventh somite. The ninth, tenth, and eleventh somites constitute the clitellum or region which secretes the cocoon. The anus lies either in the last annulus or in front of it. It may be added that papillae occur also on the sucker, but they do not afford any clue to its composition.
The grouping of the annuli, as above detailed, is characteristic of the genus Hirudo. Slight differences are observable in allied genera.
The medicinal Leech varies much in its colouration: and no less than sixty-four varieties have been enumerated. The variety H. medicinalis has in the natural state the dorsal surface greenish grey, with three rust-red longitudinal streaks on either side. The middle one of these three streaks has a black spot more or less distinct on each annulus, and it may be readily seen that one of these spots at regular intervals is much enlarged. It marks the last annulus of a somite. There is also a small black spot in the same annulus interrupting the inner light line. The ventral surface is greenish yellow spotted with black, or else black. The action of the spirit soon destroys these bright colours, as it has done here. H. officinalis has a median dorsal green band bordered by a red or brown line. The lateral dorsal regions are green with black and reddish-brown spots, sometimes grouped in two longitudinal lines. The amount of black pigment is very variable, and sometimes the red prevails. The ventral aspect is green, and as a rule not spotted.
There are many intermediate forms between these two varieties, but however much the colouration may change, the form of the teeth distinctive of the medicinal Leech remains constant (Leuckart).
The somite is not always composed typically of five annuli in Leeches. In Branchellion it has three, in Pontobdella four. The Gnathobdellidae appear to agree with Hirudo.
The ten-eyed Leeches of Japan possess six segmental papillae on the dorsal, and six on the ventral surface of the annulus that bears them.
The clitellar somites form the cocoon which contains the ova, a certain number of spermatozoa with albumen, the latter absent in Piscicola. The substance of the cocoon is secreted by the clitellar glands (infra). When it is fully formed, the animal withdraws its head, and the two ends of the cocoon close up. The openings are plugged by hardened albumen, through which the young Leeches eat their way when ready to escape. Hirudo, like Aulostoma, lays its cocoon in damp earth. The cocoon is usually attached to some foreign object in the water. Its shape is variable among Leeches.
The surface of the body is covered by a delicate cuticle perforated by pores, which are the apertures of unicellular glands. This cuticle is continually undergoing regeneration, the old one being peeled off, as may be readily seen in a Hirudo kept in confinement.