The stomach is a relatively short tube: its calibre varies and is sometimes very small. In Clepsine it has four pairs of lateral caeca. It is followed by a short intestine and a rectum which ends in an anus placed dorsally and in front of the sucker, except in Acanthobdella, where it lies in the sucker itself.

The circulatory system of the Rhynchobdellidae consists of four longitudinal vessels with muscular walls, viz. a dorsal, double in Branchellion, a ventral and two lateral, one on either side. In Branchellion a branch from each lateral vessel passes outwards into the first pair of branchial leaflets, and into every third succeeding pair. It ends by an open mouth in a dilatation which communicates with the lateral coelomic sinus. Similar but rudimentary dilatations are present in Pontobdella, Piscicola, and Clepsine. In the Gnatkobdellidae the pair of lateral vessels are alone present. The vascular system is connected with the coelome in the Rhynchobdellidae probably only in the branchial dilatations or their rudiments; in the Gnathob-dellidae with the spaces of the botryoidal tissue (p. 218), and through them with the remnants of the coelome. The coelome is much restricted by a of elongated hypodermis cells. In the eyes the vacuolated cells become very numerous; they are surrounded externally by a coat of pigment cells, and the axis of the eye is occupied by what appears to be a string of hypodermis cells detached from the cap of short hypodermis cells which covers the centre of the organ superficially.

In Clepsine, according to the same authority, there are six dorsal and four ventral rows of segmental papillae. The term goblet-organ is a misnomer. The centre of all three sets of organs projects naturally, but it may be and is depressed by their retraction. The eyes and segmental organs are, so far as can be judged, visual organs in the sense of appreciating the difference between light and shade, and they are perhaps also tactile. The goblet-organs of the lips are probably both gustatory and tactile. See p. 215, ante.

Growth of connective tissue, which splits it up into sinuses and channels, a process termed diacoelosis. This tissue may in its turn undergo meta-coelosis, i. e. become secondarily hollowed out, a process most marked in Nephelis and Trocheta (Bourne). There are four main sinuses in the Rhynchobdellidae, a dorsal, a ventral which always lodges the nerve-cord, and two lateral. The dorsal and ventral sinuses are alone present in Gnathobdellidae, and the former is sometimes absent (Nephelis, Trocheta). Coelomic epithelium is found lining the sinuses in the former group, but not in the latter. There are other remnants of the coelome round the generative organs, etc, but different Leeches vary much in these points. The blood is colourless in Rhynchobdellidae, but in the Gnathobdellidae the plasma is red with haemoglobin. There are colourless amoeboid corpuscles, and in some Gnathobdellidae at least nuclei set free from the excavated connective tissue cells.

1 See Whitman, op. cit. p. 388.

2 The caeca extend into the branchial folds of Lophobdella.

The excretory system consists in Pontobdella of a network of tubules lying within the muscular layers, continuous from side to side of the body, and provided with ten pairs of segmentally arranged nephridial funnels and as many external openings. There appears to be a similar network in Branchellion and Piscicola. The latter has also ten pairs of external apertures. In Clepsine and other Hirudinea, so far as is known, the nephridia are paired organs, independent of one another, and complicated in structure. In Hirudo and its immediate allies, the nephridial duct terminates in a contractile vesicle which opens externally. The ciliated nephridial funnel is rudimentary in Hirudo, Haemopis, Haemadipsa, or occasionally occluded as in Pontobdella. It lies in a blood, i. e. coelomic sinus; the ventral sinus in Clepsine; a dorso-ventral sinus in Pontobdella; a small isolated remnant of a sinus in Hirudo, etc.; or in a metacoelome excavated in botryoidal tissue, as in Nephelis and Trocheta. The ductules in the secreting part of the gland ramify in the gland-cells. The external nephridial aperture is ventral or marginal in Haemadipsa japonica, and on the first annulus of a somite in Pontobdella, on the last in Hirudo1.

The Hirudinea are hermaphrodite. The apertures of the male and female organs lie near one another in the median ventral line, the male in front of the female. The testes vary in number from five to twelve pairs segmentally arranged, or a much larger number not arranged segmentally as in Nephelis, and connected, each testis by a separate duct, to a common longitudinal vas deferens on either side of the body. The two vasa deferentia are either coiled or dilated at their anterior extremities, and unite in the middle line. The Gnathobdellidae have a protrusible muscular penis; the Rhynchobdellidae a short eversible sac. Attached prostatic glands are present in both cases. The female organs consist of a pair of ovaries, a pair of ducts which unite, and in Gnathobdellidae form a long unpaired portion, or utero-vagina. The ducts of both testes and ovaries develope independently of the glands in Clepsine (Nusbaum). The true ovaries of Hirndo are two solid bodies, inclosed each in a capsule which is a part of the coelome, and into which the two oviducts open. The epithelium of the ovarian sac itself produces the ova and yolk-cells of Pontob-della and the egg-strings of Nephelis. The similar strings of Clepsine probably have a similar origin.

There is a sexual congress, except perhaps in Clepsine, where it is possible that self-fertilisation occurs 1. The sperma^ tozoa are inclosed in a spermatophore. The ova are laid together with albumen in a cocoon, which is secreted by the glands of the clitellum (supra), and which is stripped off by the withdrawal of the fore part of the body of the parent. But in Clepsine the ova are united by the secretion of the glands of the ventral surface of the clitellum, and are thus attached in a mass to some foreign object in the water; the parent broods over the mass, and the young when hatched attach themselves to its body. Nephelis affixes its cocoon to water plants, but the cocoon of Hirudo and Aulostoma is laid in damp earth. Segmentation is unequal. A remarkable metamorphosis occurs in some instances (Aulostoma, Nephelis). The larval ectoderm, muscle-layers, nervous-layer (?) of reticulated cells, are thrown off, a new pharynx is formed, and with the exception of the epithelium of the mesenteron, the whole body of the future Leech is derived from paired mesodermal rudiments, two cephalic, and two for the body. There are two pairs of provisional kidneys. The young Leech has a coelome with septa corresponding to the somites.

A number of posterior somites fuse to form the sucker.

1Whilst the Leech, and more especially the Land Leech of Japan, is engaged in sucking, a clear fluid covers it and may fall in drops. It is derived chiefly if not entirely from the nephridial vesicles, and may be seen to exude from their external pores. The organs in question may therefore serve as a storehouse of liquid to moisten the skin. They are peculiarly large in the Land Leech, and the three pairs opening within the limits of the clitellum have a thick cubical epithelium disposed in folds. It is possible that they furnish some of the liquid contents of the cocoon. The epithelium of the vesicles in other parts of the body is a pavement epithelium. See Whitman, op. cit. p. 327, pp. 338-346. The Japanese Land Leech leaves a slimy track like a snail's behind it as it crawls.

The Hirudinea live for the most part in water, fresh or salt; some, however, are terrestrial, like the widely-spread Land Leeches (Ceylon, the Himalayas, Java, Sumatra, various East Indian Islands, Japan, New South Wales, Queensland, Trinidad, and South Chili), or subterranean forms such as the European Trocheta and the South American Cylicobdella, and Mac-robdella Valdiviana, which is the largest Leech known, and is said to attain a length of two and a-half feet. The majority are parasitic, and suck the blood of animals (Vertebrates, Snails); some are carnivorous like Aulostoma, Trocheta, Macrobdella.

1The somite behind the clitellum in Macrobdella, i. e. the twelfth, bears on its ventral aspect a swollen oblong area, within which open twenty-four gland pores. These glands may possibly subserve copulation (Whitman, op. cit. p. 379).

The Hirudinea are classified as follows

1.Rhynchobdellidae: body elongate, cylindrical, or flat, with well-marked anterior as well as posterior suckers; fore-part of the body retractile, and constitutes a proboscis. Piscicola, Pontobdella, Branchellion, Clepsine, etc.

2. Gnathobdellidae: no proboscis. Anterior end of the body more or less expanded; mouth sucker-like; pharynx usually armed with three jaws; blood-plasma red. Hirudo, Haemadipsa, Leptostoma, Trocheta, Aulostoma, Nephelis, etc.

See lit. pp. 216; 218; 220; 223.

Batrachobdella, Viguier, A. Z. Expt. viii. 1879-80. Lophobdellidae, Poirier and Rochebrune, A. N. H. (5), xiv. 1884.

Generative organs of Pontobdella, Dutilleul, C. R. 102, 1886; development in Clepsine, Nusbaum, Z. A. viii. 1885.

Development: Germ-layers in Clepsine, Whitman, Z. A. ix. 1886; Bergh, ibid. Metamorphosis of Aulostoma, Bergh, Arb. Zool. Zoot. Inst. Wurzburg, vii. 1885; of Nephelis, Id. Z. W. Z. xli. 1885.