LEECH. The medicinal leech (Hirudo medicinalis, L.) is a species grouped under the family Gnathobdellidx (with a dental apparatus composed of armed muscular ridges) of the discophorous Annelida. The body of a leech is spindle-shaped, and flattened dorsally and ventrally so as to be elliptical in transverse section. It is somewhat pointed in front except when the mouth is in action, while posteriorly it is terminated by a disk or sucker. The surface is marked by a series of annulations reaching from, ninety-five to one hundred, but such are only cutaneous, as indicated by the ganglia, the segmental organs, the white spots on each side, and even by the arrangement of the two outer yellowish stripes, for the primary segments of the body comprise from three to five of these. The anterior sucker (fig. 1, a) is composed of four incomplete annuli and an-other surrounding the mouth, while the posterior (a') has seven. The colour of the dorsum is gene-rally dull olive or olive-brown, with six yellowish, rusty, or greenish-yellow bands more or less interrupted with black, the spots of the latter being some-what symmetrically arranged in the two outer rows. The ventral surface is speckled with black spots on a greyish ground. Seve-ral varieties occur, according as the dorsum is lighter or darker brownish or olive, and the vent-ral surface with or without spots. Thus Moquin-Tandon, Diesing, and others indicate six or seven, each of which again has various subvarieties, ranging from two to five. Externally the body is invested by a thin translucent chitinous cuticle, which is per-forated, apparently with some regularity, by the apertures ofFlG j_ the glands. This coat is shed at intervals. Beneath is the hypoderm (epidermis of some), which is much firmer and. thinner than in the Nemerteans. It contains the pigment, though part of the latter intrudes into the subjacent layer, and is com-posed as usual of columnar granular cells, a horizontal sec-tion presenting a somewhat regularly areolated aspect. Raw-lins Johnson alludes to the vas-cularity of the surface of the leech, and Ray Lankester notes the extension of the capillaries into this layer. The latter has not been verified, even in the hypoderm of the snout, though preparations presenting such appear-ances are not uncommon. The hypoderm is closely united to the subjacent muscular layer, though it can hardly be said with Gegenbaur that it is continued into the parenchyma of the body. It is this layer and the cuticle which are marked by the superficial annulations. Various unicellular glands occur underneath the hypoderm, in particular two chief sets—superficial and. deep. The former are situated amongst the outer (circular) muscular fibres and pigment, while the latter lie amongst the con-nective tissue, muscular fibres, and vessels that constitute

the "parenchyma" between the muscular layers of the body-wall and the alimentary canal. It has been suggested that the former secrete the ordinary mucus, the latter the cocoons. Both open by ducts on the surface of the cuticle, and it is stated that those in the neighbourhood of the genital segments are enlarged at the time of oviposition. In the Nemerteans it is the homologue of the leech's hypoderm which secretes the envelopment of the ova.
The muscular layers consist of external circular fibres in several strata, between which the hypodermicglands, pigment-cells, and vessels intrude. When this coat is examined in thin superficial (horizontal) sections the fasciculi are observed to be separated by intervals. Other circular fibres occur within the longitudinal layer. The latter muscles form the great mass of the body-wall, and are grouped into various bands by the connective tissue and radiating muscles. The latter pass directly from the dorsal to the ventral surface laterally, and thus become vertical fibres; and they are very well seen in Nephelis, where they form four or five conspicuous bands between the circular layer dorsally and ventrally, and thus appear to have a considerable influence in determining the shape of the body. The extensible snout presents a muscular structure analogous to that of the tongue in the higher animals, and it is capable of even more extensive and varied movements. A complex series of muscles (circular, radial, and longitudinal) exists in connexion with the posterior sucker. The muscles of the leech are non-striated, and are formed of long spindle-cells with nuclei. The locomotion of the leech is effected by the alternate attachment of the suckers, or by swimming through the water like an eel. It is fond of waving its body to and fro in the water when attached by its posterior sucker, and this would certainly aid the aeration of the blood in the superficial vessels.
There is no special body-cavity, the blood-vessels and connective tissue alone occurring between the muscles and the digestive chamber. Bolleston speaks of dissepiments between the digestive diverticula, that between the last two not being prolonged to the ganglia. In the histology of the leech an important part is played by the connective tissue, which envelops all the organs, traverses the muscles, and is filled in certain places and in its cellular elements by brown granules. Moreover, certain of these cells are stated by Bay Lankester to form the walls of the blood-vessels.
Suctorial The inferior surface of the snout constitutes a spoon-appara- sliaped. cavity leading into the mouth, which thus with its marginal lip is capable of forming a most efficient sucker. At the junction of the buccal with the pharyngeal region are a median dorsal and two lateral prominent semicircular or sometimes slightly hatchet-shaped elevations, which in contraction fit into pits in the wall. On the free edge of each of these muscular cushions the chitinous buccal lining is furnished with a closely arranged and microscopic series of transverse processes (eighty or ninety in number), each of which somewhat resembles the middle valve of a Chiton or the upper jaw of Physa. They are arranged indeed after the manner of the ridge-tiles of a roof, the lateral pieces sloping downward on each side from the prominent median point. These angular transverse plates are sepa-rated by a well-marked interval, and they commence as small processes. They are distinctly calcified. It is these organs, mounted on the three muscular cushions, which cause the somewhat triradiate wounds, and which may pass through the true skin to the cellular tissue, a feat which Poupart's notion of suction could hardly accomplish. Great ambiguity seems to run throughout text-books on this subject, and yet the figures of Brandt and Moquin-Tandon represent the condition very fairly, though some appear to have mistaken the lateral view of the muscular cushion for a "horny jaw." These teeth can only act en masse with the muscular pad on which they rest, and have not the individual movement seen for instance in the long hook-rows of certain polychsetous Annelids. As Leuckart and others have shown, each of these muscular cushions has a most complex structure. The superficial fibres are for the most part oblique, the central vertical (that is, at right angles to the teeth) and cut into lamelte by transverse fibres. The whole forms a very efficient motor apparatus for both cushion and teeth in all their varied functions.
The mouth opens into the pharynx, the structure of Organs which, as in other Gnathobdellidss, differs essentially from of A}~ that of the Rhynchobdellidx. In ordinary contracted gestlon preparations the central canal in front is either triangular or triradiate. Internally it is covered by the cuticular and the tough hypodermic layers, from which the radiating muscles pass to the body-wall, the space between the hypoderm and the strong circular fibres of the organ being occupied by regularly arranged longitudinal fibres clasped by the radial fibres. The mixed muscular layer of the body-wall occurs outside the foregoing. The entire arrangement is well adapted for dilating, shortening, and lengthening the canal, and performing all the complex actions of a powerful suctorial apparatus. In the Rhyn-chobdellidse, on the other hand, the protrusible proboscis, with its intricate structure and its sheath, presents little in common with the foregoing. The pharynx terminates in the stomach, an elongated chamber having eleven lateral diverticula (c to c"), which form short pouches directed backward on each side, with the exception of the posterior pair (c"), which are so large and long as to be almost in apposition when distended, and nearly to reach the ter-mination of the body. From the point of bifurcation the canal proper (c) is continued as a somewhat small tube—to end in an anus on the dorsum, immediately in front of the posterior sucker. The inner surface of the alimentary canal is lined by a minutely granular epithelium. Salivary glands have been described by various authors as situated in the parenchyma outside the pharynx, and the number of large granular glands in this region is certainly great. Digestion seems to be slow in leeches, and breeders feed them with blood only once in six months. It is well to remember that the alimentary canal contains blood in those brought direct from their native marshes.
The nervous system consists of twenty-three pairs of Nerves ventral ganglia, the first being connected by commissures and (between which the gullet passes) with the supra-oesophageal g™^"g or cephalic ganglia. An intermediate stomato-gastric ganglion sends branches to the central muscular cushion for the teeth, and another on each side gives twigs to the lateral cushions. The cephalic mass supplies the eyes and the cup-shaped sense-organs. The former, to the number of ten, are situated on the three anterior segments and on the fifth and eighth segments, the whole forming an ellipse, and their structure has been carefully investigated by Leydig and others. Dr R. M. Gunn observes that in the leech they are formed of cup-shaped or bell-shaped depressions of the skin, surrounded by numerous pigment' cells. The fundus is furnished with large clear cells having peculiar nuclei. They are merely altered epithelial cells, and are found to be continuous with them. Between these in the axis of the cup is a space traversed by a nervous filament which pierces the fundus. According to Leydig this nerve-filament ends in a freely exposed papilliform elevation at the mouth of the cup-shaped eye. No connexion has been found between the nerve and the cells. Milne-Edwards, again, suggests that these refracting cellules are very like the primordial cellules of the refract-ing cone of the retinal composite eye of insects. Near the

mouth of the cup Ranke figures retinal cones (Glaskorper-kugeln), which are arranged like nerve end-organs in a mosaic, on a flat extension of the optic ganglion. These cones are very like those of the vertebrate eye, consisting of a somewhat rounded granular body, connected at the base with a nerve-filament, and having a clear, stiff, rod-like projection on its outer part. Dr Gunn has been unable to see these cones or the termination of the optic nerve. The wall of the clear cell is very thick, and the " nucleus " is generally seen to be an inward projection of this wall coding in a knob-like enlargement. Where it appears free, that is probably due to the plane of section, the side or end of the knob being severed from its connexion. Besides the cells having this inward projection of the wall there are others containing highly refractive spherules like oil globules. Ranke observes how little these " optic cups " differ from the touch or taste organs scattered on the snout and sides of the animal, and he is of opinion that they probably serve equally for the three kinds of sensory per-ception (sight, taste, and touch). If Ranke's account is correct, and if the cone-mosaic situated at the mouth of the cup be directly stimulated by the rays of light, it is difficult to account for the function of the large clear cells, and more especially the pigment around. From the position of the pigment it cannot serve for the isolation of Ranke's elements, and it can hardly be required for the prevention of the confusion of images. Yet by its presence the eye of the leech is distinguished from the adjacent and very similar touch-organs. Dr Gunn is of opinion that the light acts on the pigment, aud develops some form of energy which affects the contents of the cell, whence a stimulus is communicated to the nerve. Unfortunately a nervous connexion with these cells has not been found.
The three anterior pairs of ventral ganglia (b) coalesce
nto a single mass, and in the same way the last large
ganglion [V) is composed of seven. The ordinary ventral
ganglia give off two branches on each side, one of which
has a small ganglion developed on it. The penultimate
ganglion sends off only a single branch on each side, while
the last gives off from seven to nine for the supply of
the posterior sucker. The nerve-cells, as usual in these
ganglia, are chiefly external, and the fibrous region internal,
while the whole is surrounded by a neurilemma. This
system has been the subject of many elaborate researches,
amongst which those of Leydig and Hoffmann are conspicu-
ous. A sympathetic or azygos nerve discovered by Brandt
runs along the ventral surface of the digestive tract. In
development it is found that in many leeches the long
cords are originally separate, but afterwards come close
together so as to resemble a single connecting cord.
Circula- The circulatory system presents a median dorsal, a median
tion. ventral, and two large lateral longitudinal trunks, all
anastomosing with each other, and giving off numerous
branches to the muscular layer of the mesoderm and various
internal organs. The median sinus in the head surrounds
the ganglia and oesophageal ring. It has a ventral develop-
ment in the rest of the body, where it encloses the
alimentary canal and the gangliated nerve-cord. The
blood-vessels have a well marked systole and diastole—from
eight to ten times per minute. The fluid is red, and devoid
of corpuscles. Old observers noticed the finely reticulated
condition of the integuments when the vessels were injected,
but, as formerly noticed, vessels could not be seen in the
hypoderm proper. The active to and fro waving movements
of leeches in the water when attached by the posterior
sucker are probably connected with cutaneous respiration.
Seg- No part of the leech has caused more discussion than
mental the series of seventeen pairs of segmental organs (e, e) which organs. occur jn a line external to the testes, and alternating in posi-tion with them. Some considered them respiratory, others
excretory, while a few connected them with the reproductive system. They consist of a muscular saccate ciliated organ which communicates with the exterior near the posterior part of each primary segment, and externally of a loop-shaped gland, labyrinthine in structure, one end of which opens into the former sac, while a csecal process is prolonged on each of the testes in their region. In minute structure it has been found that the cells which constitute the gland are all penetrated by ductules, which, however, do not communicate with the large duct in the axis of all the lobes (Bourne). The gland is surrounded by an elaborate plexus of blood-vessels. These organs are in the embryo preceded, in the posterior region of the body, by three pairs of looped canals, which disappear before the permanent ones are developed.





The leech is hermaphrodite, but congress of different individuals Repro-is necessary for reproduction, and thereafter spermatophores, which duction. have a special covering, are found in the respective vaginae. The male organs consist of an intromittent apparatus (h) with a muscu-lar and glandular basal structure (g), and. a duct (/) on each side from the vesicula seminalis. The latter has a vas deferens connect-ing it with the nine globular testes (d, d', d", &c.) ranged along each side of the body, one of which is displaced outward at d". The intromittent organ reaches the exterior at the junction of the first and second sixth of the body (between the twenty-fourth and twenty-fifth rings). The female apparatus is placed in the segment between the seminal vesicles and the first testis, four annuli inter-vening between the respective sexual apertures. The external opening of this system (between the twenty-ninth and thirtieth rings) leads into an oval sac (j), the vagina—furnished with thick muscular walls. A coiled oviduct passes from its apex through glandular tissue, which probably secretes the albuminous matter surrounding the eggs, and divides into branches, one leading to each ovary (i). In Hsemopis the ovaries form a coiled filament, and on this the ovarian germs are budded. The ova are connected with the filament by a thin envelope whicn is drawn out into a stalk. There is no cord in Ncphclis, but the ovarian germs form groups of cells.
Three or four days after congress the leech may be observed to be contracted above and below the genital apertures, and an abund-ant secretion is poured out so as to surround this region of the body, as in the Nemerteans. Into this investment the contents of the female organs and their opaline gelatinous envelopment are forced. The animal elongates the anterior part of its body, withdraws its head, and the structure just mentioned slips off as a cocoon con-taining from five to eighteen ova, and frequently showing slight elevations at the points through which the body passed. The cocoons are deposited in cavities in the mud during the summer and autumn, and some seem also to deposit them during the winter. The ovoid cocoons consist of a network of spongy fibres, and indeed have been mistaken for a sponge. The older authors considered the leech viviparous until Noble and Rawlins Johnson observed the foregoing phenomena.
There is little difficulty in rearing leeches in confinement if a Leech proper method is followed, and accordingly various leech tanks and breeding, ponds have been constructed. One of the largest schemes of the kind is a leech farm of 13 acres near Newton, Long Island, U.S. The breeding ponds are in oblongs, each of 1J acres in extent, and 3 feet or more in depth. The bottom is composed of clay, and the margins of peat. The cocoons are deposited in the soft peat from June onward, the chief enemies being musk-rats, water-rats, and water-shrews, which dig the cocoons out of the peat. The adult leeches are fed every six months on fresh blood placed in linen bags suspended in the water. It is also the opinion of some that leeches which have been filled with blood make good breeders.
In regard to the development of the Gnathobdellidx, Nephelis, Develop-perhaps, has been more completely worked out than Hirudo (though ment. the observations of "Weber, Leuckart, Robin, and others on the latter are important), and, as the former very much resembles the latter, except in the presence of cilia in the embryo anteriorly, a brief notice of it will suffice. Butschli describes the usual divi' sions of the eggs, which need not be given in detail, especially as an excellent summary is to be found in Balfour's Comparative Embryology. According to these authors the cells which constitute the epiblast give origin to others which form the hypoblast and vitelline spheres. Two patches of epiblast gradually spread ovei the vitelline spheres. Then the hypoblast cells increase and fill up a space bounded behind by three vitelline spheres and in front bji the epiblast of the anterior end. At the sides of the hypoblast the mesoblast has become established, probably as two lateral bands. The hypoblast cells range themselves round a central cavity, increase, and become filled with food-yolk. The mouth and thick-walled oesophagus are then developed, probably by epiblastic invagination.

The mesoblast now forms two lateral curved bands at the sides of the body. The three vitelline spheres become covered with the flattened cells of the epiblast. The cephalic region becomes ciliated, and the cilia enter the oesophagus. The epiblast develops the cuticle, which is raised into transverse rings, without, however, having any relation to the true segments of the mesoblast. The nervous system is probably derived from the epiblast, the ventral cord breaking up into a series of ganglia, which correspond with the true somites, except that the first and last, as already mentioned, are composed of several. The supra-cesophagal ganglia arise inde-pendently. The mesoblast probably takes its origin from the two mesoblastic bands, and the segments formed by it grow upward and meet in the dorsal line, and septa are formed between the somites. The somatic layer of the mesoblast gives rise to the muscles. The mesoblast also gives origin to the excretory (segmental) and genera-tive organs, and the vascular system. A delicate musculature, how-ever, would appear to be developed independently of the mesoblastic bands. The mouth and pharynx are formed by the epiblast, the rest of the canal by the hypoblast, which from the first has a sac-like shape. The posterior sacculation of the stomach in Hirudo is originally unpaired. The dental pads are formed about the same time as the eyes as protuberances of the oral cavity. The anus is developed very late above the posterior sucker. In the embryo of Hirudo Leuckart found three pairs of segmental organs at the posterior end of the body, consisting of an enlargement from which a convoluted tube is continued for some distance backward, and then bends forward to open on the exterior. The anterior part is broken up into a labyrinthic network. These organs disappear in the adult. The recent researches of Whitman on Glcpsine and of Hoffmann have greatly extended our information with regard to the histology and morphology of the parts in the embryos of the leeches.
The time between the deposition of the ova and their hatching is variable, and probably depends, as in the ova of the Salmonidss, on temperature and other causes. It is said to range from twenty-five to forty days. The young arrive at perfect coloration when two years old, and become sexually mature at three years, about which age they become fit for medicinal use; their food consists at first of microscopic organisms, and afterwards, when the mouth has attained more complete development, of the larvai of insects and other small animals.
There is no annelid that has been more prominently brought under notice than the leech, both on account of its use in medicine from very early times, and its fitness for anatomical and other in-vestigations. The number of treatises, inaugural, historical, and structural, that have been devoted to it is very considerable; of these the voluminous article in Brandt and Batzeburg's Medicin-ische Zoologie may be taken as a type.
Medici- The leech is the BSiWa of Herodotus, Theocritus, nal use. Nicander, and other Greek authors, and the Hirudo and Sanguisuga of Plautus, Cicero, Horace, Pliny, and other Roman writers. Cselius Aurelianus mentions its use, and Galen and his successors recommend its application. Appiau also alludes to the latter, and describes very graphically the process by which it fills itself with blood. It was sufficiently familiar to naturalists both before and after the time of Linnaeus, though occasionally there has been considerable ambiguity in regard to species. The use of the leech is mainly for local blood-letting, but in modern times the practice has greatly diminished ; indeed, in some cities the druggists chiefly use them with doubtful efficiency in cases of incipient gumboil and in facial ecchymosis. They may be applied to any part of the adult skin, and to the mouth, fauces, and other available inlets by the aid of a leech-glass, which consists of a tube with a slightly con-tracted aperture, and provided (or not) with a glass piston to push the leech onward. In China a piece of bamboo serves the same purpose. For such functions the most active specimens should be chosen (and, as Sir Robert Christison states, these contract firmly when squeezed in the hand) and kept for an hour out of water, and then applied to a perfectly clean surface of skin. They may also be made to bite by smearing the skin with cream or blood, or by immersing the leech for a minute in porter or tepid water. Each fills in about fifteen minutes, and draws from 40 to 85 grains of blood, or, including that afterwards obtained by fomenting the wound, about half an ounce. In young children they should never be placed on parts where firm pressure cannot be applied. It was formerly the practice to prepare the leeches that had been used for further action by sprinkling a few grains of salt on the snout, and stripping them gently between the fingers so as to cause them to eject the blood This plan is not now adopted, and rightly so, since various diseases might thus be communicated. They certainly can be applied four and five times in succession by placing them in vinegar and water, and afterwards in a vessel (which the French call a domestic marsh) with turfy earth; but they draw less blood on the fifth occasion. Should the haemor-rhage from the wounds (as in certain constitutions) prove severe, it may be staunched by the application of vinegar, solid nitrate of silver, a hot wire, or a hot solution of alum, or by acupuncture. If a leech by accident be swallowed, a pretty strong solution of common salt, or a glassful or two of wine may be taken. Instead of the actual leech an instrument called an artificial leech is now sometimes used. This consists of a small sharp steel cylinder (worked by a spring) with which a circular incision can be made through the skin, and a glass cylinder capable of being exhausted by a piston worked by a screw. Care must be taken to move the piston at about the same rate as the blood flows, and the edge of the glass cylinder should not press too tightly, else the flow is arrested.
Leeches are imported from France and Hungary, and also through Hamburg from Poland and the Ukraine ; they likewise come from Turkey, Wallachia, Russia, Egypt, and Algeria. They are found in Britain—both in Scotland and England, but especially in the latter. In the French trade Bordeaux leeches are preferred ; Polish, Swedish, and Hungarian are those most commonly met with in Britain. It is difficult to estimate the number of leeches now used. In 1846 Moquin-Tandon calculated that there were from twenty to thirty millions used in France; and Leuckart mentions in 1863 that in London seven millions, and in the Parisian hospitals five to six millions, were annually employed At the great American leech-farm the average sale is one thousand per day. There cannot be a doubt, however, that the use of leeches at the present time is greatly restricted—indeed, the younger generation of British medical men seldom or never prescribe them—so that scarcely one will now be employed where one hundred were a quarter of a century ago. This is very well shown in a note from Messrs Duncan, Flockhart, & Co. of Edinburgh, from which it appears that the account for leeches supplied during three months in 1844 to the Royal Infirmary, Edinburgh, was £45. This steadily decreased until about 1868 it amounted for the same period only to 5s. 6d. Sir Robert Christison mentions that the price of the best leeches in 1845 ranged from £4 to £8 per thousand; twenty years ago they were from £10 to £15 per thousand; and at the present time good leeches cost about 10s. per hundred, or £5 per thousand.
They inhabit ditches and ponds, with pure running water, weeds for shelter, and muddy banks and bottom. They are captured by nets after attracting them by baits, or by wading into the water, and then stripping them off the legs on coming to land. Leeches are preserved in loose turf or moss constantly moistened, or in earthenware or glass vessels half full of water, covered with glass or linen-gauze; and some place a rusty nail, others a clean sponge in the vessel, which can be exposed to the light. In transporting them the French " domestic marsh," a vessel with small perforations interiorly and filled with moist turfy earth or peat made into a stiff mud, is excellent. Sometimes an exterior vessel with a few inches of water is placed round the former. The mouth of the vessel is closed with a coarse linen cloth. Leeches, like many other annelids, live for several years without food in vessels of pure water.





The group (Hirudinei or Discophori) may be divided into three cation, families, viz., Ehynchobdellidse, Gnathobdellidse, and Branchio-bdellidee.
The Ehynchobdellidse are those leeches furnished with a pro-trusible proboscis (which is often exserted if the animal is removed from the water and placed on a dry surface). This family includes the fish-leeches (Ichythyobdcllidse), which have an anterior and posterior sucker, a simple intestine, and mostly two pairs of eyes. Amongst these are Piscícola geómetra, L., found on freshwater fishes, P. hippoglossi on the holibut, and P. respirans, in which the body has lateral sacs into which the blood enters. The first-mentioned (P. geómetra) is a somewhat beautiful species, and full of activity, waving its body to and fro, and floating by aid of the expanded posterior sucker on the surface of the water. Another well-known genus (Pontobdella) is characterized by its thick warty skin, and four rings to each segment. The best-known example is the skate-leech (P. muricata, L.), which is olive-coloured and dusted with whitish grains. The anterior sucker is furnished with papillae round its edge. It adheres to the skin of the skate, and deposits the curious pedicled horny capsules, containing a single egg, inside shells. In the same group is the remarkable genus Branchellion, which has a narrow nuchal region with the sexual orifice at its posterior part, and a scries of frilled lateral appendages, the function of which has been supposed to be branchial. Its stomach is saccu-lated. One species (B. torpedinis, Sav.) is a messmate of the torpedo or electric ray of the Mediterranean ; this has been the subject of very interesting papers by Leydig and De Quatrefages. The next subfamily—the Clepsinidee,—have somewhat broad bodies capable of being curved downward at the margins so as to form a hollow ventral groove for the lodgment of the eggs and the young, while the snout is pointed. They have from one to four pairs of eyes, and three rings to each segment. The dorsal blood-vessel is rhythmically contractile, and the median blood-sinus envelops the digestive canal and the ventral nerve-cord. The stomach is branched, and the anus opens above the posterior sucker. The skin in many is warty, and in the Clepsine echinulata of Grube, from Lake Baikal, the dermal papilla? are furnished with soft pointed processes, so that in outline they are spinulose. The oviducts have no common tract or vagina, but open at the female pore. The genital apertures occur between the twenty-fifth and twenty-sixth, and between the twenty-seventh and twenty-eighth rings. The ova are in some kept under the body till hatched. Several species abound in the freshwater lakes and ponds of Britain, and their remarkable and beautiful anatomical structure is yet in need of elucidation. Amongst those most commonly met with is Clepsine Uocidata, Sav. (fig.. 2), which is about an inch in length, generally

Pig. 2. Fig. 3. Fig. 4.
FIG 2.—Clepsine bioculata, Sav., and young. Dorsal view. Enlarged. FIG. 3.—Clepsine eomplanata, Sav. Dorsal view. Enlarged. FIG. 4.— Clepsine heterodita, L. Dorsal view. Enlarged.
has a greenish-grey hue, and is much tapered anteriorly. Two closely approximated eyes occur in front. There is a reddish-brown body on the eleventh ring, marking an.aperture described by 0. F. Miiller, and a whitish opacity in front of it. It often fixes itself by the posterior sucker, and waves its body to and fro in the water, and it swims actively like a Nemertean or horse-leech. The ova and young are carried in groups on the abdominal surface. It contracts itself into a ball on irritation. Its food consists of fiuviatile and lacustrine mollusks, especially of Physse (bubble-shells). Clepsine eomplanata, Sav. (fig. 3), again, is distinguished by its greyish-green or pale brown appearance, often with two (sometimes four) interrupted dark brown bands along the middle of the dorsum, in which are pale papilla?, four rows of the latter being generally present. The eyes are six in number, in parallel series. The body is firm, and the crenatures at the sides are never obliterated. There are six gastric sacs on each side ; and in the young the rectum is ciliated. The proboscis is a cylindrical organ slightly narrowed anteriorly and posteriorly, and finely barred with transverse stria?, a feature in C. bioculata due to the arrangement of the granular nucleated glands on its inner surface. It feeds on Planorbis and Limnmus (coil and mud-shells). Clepsine heterodita, L. (fig. 4), a somewhat smaller form, is characterized by its translucent yellowish aspect. The dorsum is rather regularly dotted with pale brownish, so as to give it a checkered appearance. The snout is acute, and is furnished with six eyes, the anterior pair being closely approximated, while the two succeeding are separated by an interval from the foregoing, and the eyes in each pair are at a greater distance from each other. The digestive ca?ca are beautiful objects from their regularity and complexity. The ova are carried on the under sur-face of the body. It is less active than C. eomplanata. Another form very abundant under flat stones in similar lakes and ponds in certain places is Clepsine tessulata, 0. F. Miiller (fig. 5), a large

Fig. 5. Fig. 6.
FIG. 5.—Clepsine tessulata. 0. F. Miiller. Dorsal view. Somewhat enlarged. FIG. 6.—Clepsine tessulata, 0. F. Miiller. With a swarm of young adhering to the ventral surface. Slightly enlarged, as adhering to a glass vessel.
and conspicuously tinted form. It reaches the length of 3 inches, and is of various shades of green, brownish, or olive, with six rows of yellowish or whitish specks, the marginal in all cases being the largest, while the four internal occupy papilla?. The eyes are eight in number in two series approximated in front. The soft, mobile, and almost gelatinous body is capable of assuming endless shapes, and is sometimes like a cordate leaf. It is gregarious in confinement. When a specimen is detached from its own adherent mass of ova, it occasionally selects another group and fixes itself to the glass to nurse them. The young are borne on the ventral surface (fig. 6). The genus BXsementaria has two eyes, a bifid anterior sucker with the mouth in front, a long pointed pro-boscis, and five rings in each segment. The species (e.g., H. mexi-cana and H. officinalis) occur in the Mexican lakes and South America, the latter being used medicinally, since it is capable of penetrating the skin with its pointed proboscis.
The second family, Gnathobdellidae, includes the medicinal leech, besides Hirudo interrupta (M. Tand.) from Algiers, H. jaranita from Java, S. sínica (Blainv.) from China, H. quinquestriata(Schm.) from Sidney, and others to be subsequently mentioned. H. decora (Say), the native leech of North America, is used in the same way as H. medicinalis. It is bluish, with about twenty-two reddish points on the dorsum and a lateral series of black touches of the same number. The ventral surface is ruddy with black points. It also comprises the genera Bdella, without denticles, and Hsemopis, the best-known example of which is H. vorax, M. Tand., a kind of horse-leech which is very troublesome to horses, cattle, and camels, by entering their nostrils when drinking ; and the same disagreeable accident occurred to the French soldiers in Egypt. The common horse-leech (Aulastomumgulo, Moq. Tand.), with very slightly developed

lateral cteca of the stomach, but with two long posterior caeca,
is abundant in British ponds and lakes, as also is Nephelis vul-
garis, L. (fig. 7), a species about 3 inches in length. Its dorsum
is brownish-yellow, often with a conspicuously tesselated appearance,
while the under surface is pale olive. The eyes are eight—four
being placed somewhat in a semicircle, and four a short distance
behind, wider apart, and at a different angle. It is active and rest-
less, keeping up an undulating motion of its body when attached
by the posterior sucker, apparently as in the Phyllodocidce, to pro-
mote respiration. It also swims on edge through the water like an
eel. The skin is exceedingly sensitive to the vapour of chloroform,
while the contact of a single drop causes tetanic convulsions, and
the annelid dies, cpuite rigid. The digestive canal is nearly simple,
and there are no buccal teeth. The dorsal blood-vessel is absent.
The ova are deposited in a horny capsule fixed horizontally to sub-
aquatic structures, and it is curious that Linnreus described it as a
hemipterous insect under the name of Coccus aquaticus. On
Bergmann's paper in
which the error was cor-
rected the great Swede
wrote '' Vidi et dbstu-
pui." Ncphelis feeds on
earthworms, larvie, mol-
lusks, and other organ-
isms. Trocheta sub-
viridis, Dutrochet, is
a large European form
(7 inches in length),
which frequents the
marshes and ditches of
France and Algeria (and
also rarely, apparently
from introduction, of
England). It leaves the
water to follow the
earthworms on which it
feeds. There arc no
buccal teeth, and the
alimentary tube is only
slightly camerated. In
Ceylon the Hirudo
tagalla or ceylanica, a
land-leech about an inch FIG. 7.—Nephelis vulgaris, L. Dorsal view,
in length, is a great Slightly enlarged,
annoyance to travellers, especially in the rainy season, attacking men and horses when journeying through the woods and jungles, and causing considerable irritation from its bites. They come in troops out of the grass and dead leaves, and one cannot leave the gravel in the gardens in some places without being attacked. Leech-gaiters, therefore, are worn by many residents for protection. A similar form occurs at an elevation of 4000 feet in the Philip-pines, and others in Java and Sumatra; and Sir Joseph Hooker found them at a height of 11,000 feet on the Himalayas. Land-leeches also exist in Australia, Japan, and Chili,—where very few occur in the water. They frequent plants, trunks of trees, and shrubs, as well as grass. An eyeless leech, called Typhlobdella, in-habits the subterranean waters of the Baradla cave in Hungary. An allied eyeless form, Cyliobdellalumbricoides, Grube, which was found by Fritz Miiller in Brazil, lives in damp earth. It has a slender spindle-shaped outline. The exact position of the gigantic Macro-bdella valdiviaiia of Filippi, a South American leech measuring about 2J feet, is uncertain. It is eyeless, and has neither lips nor teeth. It probably lives in damp earth, and feeds on earth-worms.
In the third family, Branchiobdellidae, the irregularly annulated body is elongated, somewhat cylindrical, with a bilobed eyeless snout, and a sucker at the posterior end. There is no proboscis, but the pharynx has two flattened edentate pads (dorsal and ventral). The body is provided with a ccelom or body-cavity, an unusual feature in the leeches. The alimentary canal is simple. There are only two longitudinal vascular trunks—a dorsal and a ventral, the former showing a dilatation behind the cephalic branches, some-times termed a heart. Two pairs of segmental organs are present, die posterior pair of which are modified for the conveyance of the ovarian products to the exterior ; for the ovaries, which are situated far back, discharge their contents into the body-cavity. The best known are Branchiobdellaaslaci, Odier,and B. parasita, Henle, which occur as ectoparasites—the former (smaller) on the branchiae, the latter under the tail and on the antennee and eyes, of the crayfish. Myzobdella, Leidy, and Temnocepliala, Gay, are allied forms. The latter is a curious Chilian leech having five digitate processes at-tached to its anterior end, behind which a pair of eyes and the mouth are situated. A sucker exists posteriorly. In the same family are placed the aberrant types Acaiithobdella and Histriobdclla. The former is characterized by a somewhat flattened spindle-shaped body resembling a Gephyrean, bluntly pointed in front, furnished with minute hooks near the anterior end, and a posterior sucker.
The A. paledina, Grube, a fish-parasite from Sicily, is an example. The latter (Histriobdellidce) are remarkable in the group in being dioecious instead of hermaphrodite, and somewhat resemble in out-line grotesque insect-larvae. The peculiar beak-like head fitted for suction, the jointed body, and the pair of posterior suckers are characteristic. They are ectoparasites on marine Crustacea ; thus Histriobdclla homari, Van Beneden, occurs on the lobster, and Saccobdella on other decapods.
Formerly Udonella and Eutobdella were included under the leeches, but they seem to be more correctly located amongst the Trematoda. Until lately Malacobdella was also considered one of the group, but its ciliated skin, separate nerve-cords, proboscis, and development point it out as an intermediate type allied to the Nemerteans.

The following works amongst others may be referred to for more detailed
accounts of the order:—Noble, On the Medicinal Leech, 1822; Rawlins Johnson,
On the Medicinal Leech, 1825; Brandt and Ratzeburg, Medicinische Zoologie,
1829 ; Moquin-Tandon, Monographie de la Fam. des Hirudinees, 2d ed., Paris,
1846; E. Leuckart, Parasiten des Menschen, vol. i., 18G3 ; Sir J. G. Dalzell, Powers
of the Creator, vol. ii., 1853 ; G. Johnston, Catalogue of Worms, British Museum,
1865. Also the various memoirs of Carena, M. Thomas, Dellc Chiaje, Gratiolet,
H. Rathke, Van Beneden, F. Leydig, E. Grube, Kinberg, Robin, Vaillant, Dorner,
Kennel, Schneider, Hoffmann, Hermann, Whitman, Bourne, Ray Lankester, and
Eanke. (W. C. M.)