Dissected so as to show its reproductive and segmental organs or nephridia in situ.
A BLACK bristle has been passed into the pharynx through the nerve-ring, and the ventral chain of ganglia is visible throughout the greater part of its extent. In the middle line, covering the sixth ganglion of the ventral chain, is seen a globular body, projecting chiefly to the left side of the nerve-cord and connected posteriorly with a median siphon-shaped muscular tube. The globular organ has walls partly muscular, partly glandular, and is called consequently the prostatic part of the male intromittent apparatus. The glands appear to secrete the material which forms the spermatophores. The median siphon-shaped tube is the copulatory organ or penis, and its walls contain both circular and longitudinal muscle fibres. From the base of the prostatic body passes to right and left a ductus ejaculatorius. These are each connected respectively to what is easily seen with attention to be a mass of coiled tube of a yellowish colour. The coiled tubes, according to Leuckart, contain at the height of the reproductive season numberless minute globules. The masses in question may be regarded provisionally as vesiculae seminales. Each coiled tube is continuous with a duct, the vas deferens, which passes backwards parallel to the nerve-cord. It is slightly tortuous.
From its inner or median side nine branches arise, each passing to a testis. The nine pairs of testes are globular bodies lying close to the ventral nerve-cord, one behind each ganglion from the eighth to the sixteenth inclusive. They are therefore segmentally arranged. The prostatic apparatus, copulatory organ, and vesiculae lie in the tenth somite (Whitman), the male aperture being median and ventral in the second annulus of that somite. It is difficult to see unless the penis is protruded, as it sometimes is in this Leech and in the common Horse-leech, Aulostoma gulo, when the animal is killed by chloroform. The female organs lie in the eleventh somite (Whitman), therefore in the somite interposed between the first pair of testes and the male intromittent apparatus. Close behind the seventh ventral ganglion may be seen two roundish bodies, the capsules which contain the true ovaries, lying one on either side of the longitudinal nervous commissures. The oviduct is continuous with or rather perforates these capsules. Its anterior part is forked, one branch of the fork passing under the nerve-commissure to the left ovarian sac. The posterior part is single and may be distinguished by its yellow colour.
It enters the base of an oval sac, the vagina, which has muscular walls and a cuticular lining and opens by a median ventral aperture in the second annulus of the eleventh somite.
Externally to each vas deferens and alternating in position with each testis, is a row of globular sacs only a very little less in size than the testes themselves. These are the vesicles of the segmental organs or nephridia. Each vesicle opens by a pore on the last annulus of its somite. In front of it, and in part externally to it, is a loop-shaped body. This is the chief part of the nephridium, representing the main and apical lobes of that organ. A narrow caecal process passes inwards from it and lies upon a testis. This is the testis-lobe which ends in the nephridial funnel or nephrostome. The funnel lies in a vascular sinus, the perinephrostomial sinus, or the moniliform heart of Brandt. The two pair of nephridia lying behind the last pair of testes in the twenty-first and twenty-second somites (Whitman), possess these sinuses as well as funnels, but this is not the case with the five first pair of nephridia which correspond to the somites six to ten (Whitman). There are in all seventeen pairs of these organs.
The azygos and median position of the generative pores is a noteworthy feature, as is also the development of a muscular intromittent organ. It is doubtful however whether they are of any importance in determining the affinities of Hirudinea.
The ductus ejaculatorius, according to Leuckart, has a stratum of circular muscle fibres in its walls. The vas deferens is surrounded by a space or sinus packed with cells, 'which possess a rather degenerate appearance' (Bourne). The sacs in which the true ovaries are lodged contain similar amoeboid corpuscles. It has been suggested by Gibbs Bourne that these spaces represent a portion of the original coelome, and the cells original blood corpuscles, and that they have been closed in by the growth of the connective tissue before haemoglobin appeared in the blood-plasma.
The true ovary is a filamentous body, 10 mm. long in some instances. It is coiled within the ovarian capsule. The oviduct opens into the capsule, but it is not simply continuous with the walls of that space. On the contrary, its anterior end is disposed in coils within it. These facts support the view quoted above from Gibbs Bourne as to the nature of the capsule. The median or single portion of the oviduct is surrounded by glands, by which the albumen mixed with the ova in the cocoon is secreted in all probability. The vagina has muscular walls, is lined by cuticle, and receives the spermatophores in congress. These bodies are stated to contain not only spermatozoa, but corpuscles similar to those found in the coils of the vesiculae seminales. They are resolved in the vagina, and the spermatozoa, now free, are said to penetrate into the ovarian capsules.
The ovarian capsules are sometimes of great length, as in Nephelis and Clep-sine. In these genera 'egg-strings,' produced by the continuous division of a cell, lie free in the capsular cavity. The formation of the string from a ridge of the epithelium lining the capsule has been observed in Nephelis. The ova sometimes degenerate, and Schneider states that they are destroyed by amoeboid cells in the ovarian capsule. The same thing occurs with the spermatozoa. But the full account of his observations on these points, and on the origin of the ova in various Leeches, has not been accessible to me. Joseph has recently discovered in Clepsine that the vasa deferentia and oviducts arise independently of the sexual glands. In Branchiobdella, (which is probably an Oligochaete), both ovaries and testes are proliferations of cells lining the coelome, and the generative products are carried away by ducts with open mouths, which are, perhaps, modified nephridia. It is quite possible that the glands and ducts have the same origin in the Hirudinea. The extension of the vas deferens through several somites, and the presence of nephridia in the same somites, creates a difficulty for this view, - the same difficulty, however, that recurs in the Earthworm.
The Rhynchobdellidae possess neither the tubular intromittent organ nor the muscular vagina.
The nephridial funnel of Hirudo appears to be degenerate. It is imperforate and multilobed. The lobed ciliated cells which compose it are set upon a vesicular dilatation containing a debris of cells. The various lobes of the nephridium are made up of nucleated cells, varying in size and character. These cells are perforated by intracellular ductules with independent walls. The ductules pass from one cell into another, and are branched, especially in the cells of the main lobe, some of the branches remaining caecal. The main duct has cellular walls, its lumen perforating the cell, as in part, at least, of the Earthworm's nephridium. The vesicle has thin walls with muscular fibres, and is contractile. It is lined by a ciliated epithelium. The cells of the gland are surrounded by a rich network of capillary vessels connected with the lateral blood-vessel, and through the testicular sinus with the main ventral blood sinus. The whole gland is invested with vaso-fibrous tissue.
Nephridial funnels appear to be present in all Leeches. They vary in complexity. Among the Gnathobdellidae, they are perforate in Nephelis and Trochaeta, and in these genera they open into special spaces developed in the botryoidal tissue, termed by Gibbs Bourne 'metacoelome.' They are present and usually perforate in Rhynchobdellidae. In Clepsine they open into the ventral blood sinus, and in Pontobdella into a dorso-ventral sinus. In all Leeches the dilatation following the funnel appears to be present. As to the gland, in Pontobdella, Branchellion, and Pisicola, the tubules form a network continuous on both sides of the body and across the ventral median line. The funnels and external openings, however, of Pontobdella are metamerically arranged. Branchellion and Piscicola require further examination. There is no terminal vesicle in Rhynchobdellidae. The Gnathobdel-lidae in general appear to agree more or less closely with Hirudo.
Gibbs Bourne has found in the ductules of the nephridium in the medicinal Leech minute clear structureless bodies; in the liquid of the vesicle, and sometimes in the main duct, bunches of needle-shaped crystals, soluble in nitric acid.
According to Leuckart, the embryo of Hirudo has four pairs of nephridia in front of the first persistent pair of the adult, and three pairs behind the last persistent (or seventeenth) pair.
Genitalia. Leuckart, Die Parasiten, (ed. 1.) i. p. 672. Ovary and ova of Nephelis Aulostoma, Pisciola, Pontobdella. Schneider, Das Ei und seine Be-fruchtung, Breslau, 1883. Egg-strings of Clepsine, Whitman, Q. J. M. xviii. 1878; of Nephelis, Jijima, Q. J. M. xxii. 1882.
Sinus of vas deferens and the ovarian capsule. Gibbs Bourne, op. cit. p. 473. Spermatophores. Cf. Robin, A. Sc. N. (4) xvii. 1862.
Nephridia: of Hirudo, Gibbs Bourne, Q. J. M. xx. 1880; xxii. 1882, p. 337; of other Leeches, Id. Q. J. M. xxiv. 1884, p. 478; Schultze, A. M. A. xxii. 1883.