The testes arise as local proliferations of the epithelium of the lateral coelomic cavities, but from the somatic wall, not from the splanchnic, as in the case of the ovaries to be described later. A portion of the tissue which is to form the testis grows out laterally into a thin cord, which is to become the vas efferens of that testis. Later both testis and duct become hollowed out with a common cavity. The main portion of the vas deferens of each side, as well as the terminal copulatory apparatus, is an ingrowth from the epidermis which meets the downgrowths from the testes.
That there are considerable differences between the reproductive organs of the leeches and those of Oligochaeta will be apparent from the above description. There are, however, to begin with, certain obvious similarities. In the first place, the origin of the reproductive glands is identical; in both groups also the efferent ducts consist of two portions—an invagination from the outside, and a formation of the proximal part of the ducts near to the glands. In Moniligaster, where—exceptionally—the testes develop on the posterior wall of their segment in close contact with the funnels of the sperm-ducts, there is no very hard and fast line to be observed between the tissues of the two. The hollowing out of the testis in the leech, and the continuity of the cavity thus formed with the duct, is a specialty of the leeches not found among the Oligochaeta.
Like many Oligochaeta, the leeches may form spermatophores in which the sperm is packed for its conveyance from one individual to another. Glossiphonia (Clepsine) plana, where the structure in question has been elaborately described by Whitman,[[460]] may be selected as an example. The spermatophore (Fig. 209) is about 8 mm. long, and is clearly formed of two halves, each of which is formed separately in one ductus ejaculatorius, the soldering together being effected in the common part of the male ducts, where also a basal portion with a single lumen is added. The spermatophore has a double wall. It is deposited not in the neighbourhood of the generative pores, but upon the back; and Whitman has discovered the extraordinary fact that the spermatozoa find their way through the body-wall of the leech into the interior of its body, where fertilisation presumably occurs.
Fig. 209.—Spermatophore of Glossiphonia plana. × 22. (After Whitman.)
Female Reproductive Organs.—The ovaries of the Hirudinea appear to differ from those of the Oligochaeta in that the ovaries are continuous with their ducts. In Hirudo, however, the real ovary of each side consists of masses of germinal tissue lying freely within a sac which communicates with a duct; the two ducts unite to form a much convoluted tube which opens into a thick-walled vagina, itself opening again on to the exterior by a median unpaired opening on the seventh segment. The muscular vagina is not always present.
The median unpaired female aperture offers now no particular difficulty, since in many earthworms, e.g. Perichaeta, this orifice is in the same condition; nor does the fusion of the oviducts and the so-called ovaries; for in Eudrilus, for example, and in many Eudrilidae, the ovary is contained in a sac into which the oviduct also opens. It will be noticed too that the existence of short oviducts as compared with the long sperm-ducts is a further point of likeness to at any rate the higher Oligochaeta. But a further comparison needs first to be based upon a consideration of the development of the different sections of the apparatus in the leech. The independence of the ovaries and their ducts has been proved by several observers; quite recently Bürger has dealt with the matter in Nephelis, Hirudo, and Aulastomum gulo.[[461]] He has found that the ovaries arise from the splanchnic wall of the lateral coelomic cavities; they are therefore proliferations of the coelomic epithelium, as in Oligochaeta and all Coelomates so far as is known. The peripheral layer of the mass of indifferent cells which constitutes the ovary becomes somewhat modified; its cells are flattened, and it at length separates itself and forms a capsule surrounding the other cells, which are in fact, or become, the ovary. This capsule meets and fuses with the ducts, which are invaginations from the exterior of the body.
There are clearly differences between the ovary of a leech and that of a typical Oligochaete like Lumbricus. The only point of agreement, in fact, is the origin of the reproductive gland itself from the walls of the body-cavity. In Lumbricus and allied forms, whatever may be held with regard to their homologies, the oviducts as a matter of fact appear first as funnels, which afterwards bore their way to the exterior. They are purely mesoblastic structures, not—except perhaps the very extremity—derived from an ingrowth from the epidermis. We have, however, other Oligochaeta in which there are closer resemblances to what has just been described in the Hirudinea. In more than one point the aberrant family of the Eudrilidae come nearer to the Hirudinea than any other Oligochaeta, in spite of Branchiobdella with its jaws and sucker. Now in Eudrilus the ovary is enclosed in a capsule which becomes continuous with the duct of the great sperm-holding pouch, itself an invagination from the exterior; there is no reason in this Annelid why the ova should not reach the exterior by this system of ducts, although there is no actual experimental proof that they do so. In any case there is also an oviduct corresponding to that of Lumbricus, which opens into the opposite side of the duct of the spermathecal pouch on the one hand, and into the receptaculum ovorum on the other; it has no direct connexion whatever with the sac containing the ovary. If we were to cut off the receptaculum and the oviduct and reduce the spermathecal sac to its duct, the result would be much the same as we find it in the leech.
Cocoon.—The practice of forming a cocoon for the shelter of the eggs and of the developing young is shared with the Oligochaeta; but not all leeches deposit their eggs in this manner. Glossiphonia, for instance, carries its eggs upon the ventral face of the body, where the young remain for some time after they are hatched attached by the posterior sucker to their parent's body, and from which situation of safety they make short excursions. Other leeches deposit their eggs singly, but agglutinated together upon stones, etc. In the medicinal leech the cocoon is ovoid in shape, and from the end, which is closed by a temporary plug, the young when ready escape. This cocoon is deposited, as is that of an earthworm, in soil near to the borders of a marsh or pond, so that the young, while enjoying the requisite degree of moisture, may not be injured by a too wet environment. On the other hand Pontobdella and some other leeches lay their cocoons attached to bodies actually submerged in the water. The cocoon is secreted, as in Oligochaeta, by the clitellum, and as in them, is drawn off over the head, the ova and sperm probably flowing into it during the process. The elasticity of the slightly hardened mucus causes the two ends of the cocoon to close up when it is free from the body; it is then whitish and soft, and the leech fixes it, and appears to polish the surface with its buccal sucker. In a few hours the cocoon becomes amber brown, a colour which characterises the cocoons of a great many earthworms and other Oligochaeta. The medicinal leech forms its cocoon in the same way as does Nephelis, to which the above description refers; but when the cocoon is formed the leech covers it with another layer of mucus, which Vaillant, from whose work the foregoing notes are extracted,[[462]] thinks may be produced from the so-called salivary glands.
Classification.—The number of different kinds of leeches is at present uncertain. Seeing that no less than sixty-four varieties of the common Hirudo medicinalis—colour varieties, it is true—are said to exist, it is not wonderful that the labours of some systematists have been severe, and have provoked much criticism and alteration on the part of others.[[463]] As to genera, Vaillant, in his recent continuation of de Quatrefages' "Annelés" in the Suites à Buffon, which includes the literature up to the year 1886, allows thirty-seven, some (three) of which, however, are admitted to be incertae sedis. Blanchard, who has paid a great deal of attention to the group, reduces these by six, which he considers to be synonyms; but on the other hand he has added or rescued from oblivion six or seven others, and Whitman has instituted several Japanese and Australian genera. Most of these generic types are, however, only imperfectly known, and from external characters only. It is quite problematical how many valid genera should be retained; in the meantime those that are fairly well known are divided by Blanchard[[464]] in the following way:—