It contains the pigment, though part of the latter intrudes into the subjacent layer, and is composed as usual of columnar granular cells, a horizontal section presenting a somewhat regularly areolated aspect. Rawlins Johnson alludes to the vascularity 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 appearances 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 connective 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, pigmentcells, 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 Xephelis, 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. Rolleston 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 Ray Lankester to form the walls of the blood-vessels.
The inferior surface of the snout constitutes a spoonshaped 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 separated 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 polychaetous 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 lamellae 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 which, as in other Cnathobdellidae, differs essentially from that of the Phynchobdellidae. In ordinary contracted 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 Bhyη- choljdellidae, 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 termination 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 ventral ganglia, the first being connected by commissures (between which the gullet passes) with the supra-cesophageal 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 pigmentcells. 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 refracting cone of the retinal composite eye of insects. Near the mouth of the cup Ranke figures retinal cones (Glaskörperkugeln), 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 ending 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 perception (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. F rom 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, and 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 (δ) coalesce into a single mass, and in the same way the last large ganglion (δ') 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 conspicuous. 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.
The circulatory system presents a median dorsal, a median 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 development 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.
No part of the leech has caused more discussion than the series of seventeen pairs of segmental organs (e, e') which occur in a line external to the testes, and alternating in position 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 loopshaped gland, labyrinthine in structure, one end of which opens into the former sac, while a caecal 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, hut congress of different individuals is necessary for reproduction, and thereafter spermatophores, which have a special covering, are found in the respective vaginae. The male organs consist of an intromittent apparatus (Ae) with a muscular and glandular basal structure (y), and a duct (f) on each side from the vesicula seminalis. The latter has a vas deferens connecting 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 intervening between the respective sexual apertures. The external opening of this system (between the twenty-ninth and thirtieth rings) leads into an oval sac (∕)> 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 (ι). In Haemopis 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 which is drawn out into a stalk. There is no cord in Nephelis, 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 abundant 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 containing 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 proper method is followed, and accordingly various leech tanks and 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 1⅜ 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 Gnatliobdellidae, Neρhelis, perhaps, has been more completely worked out than Hinido (though 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 divisions 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 over the vitelline spheres. Then the hypoblast cells increase and fill up a space bounded behind by three vitelline spheres and in front by 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 independently. 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 generative organs, and the vascular system. A delicate musculature, however, 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...
