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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 hypodermic glands, 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. 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.
Suctorial apparatus.
The inferior surface of the snout constitutes a spoon- shaped 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 polychætous 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 lamellæ by transverse fibres. The whole forms a very efficient motor apparatus for both cushion and teeth in all their varied functions.
Organs of digestion.
The mouth opens into the pharynx, the structure of which, as in other Gnathobdellidæ, differs essentially from that of the Rhynchobdellidæ. 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 Rhyn- chobdellidæ, 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 (<: ) 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.
Nerves and sense-organs.
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-œsophageal 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
