Sometimes, as in our common earthworms, the sperm-duct opens directly on to the exterior of the body, the lips of the external orifice being swollen by the development of cutaneous gland-cells. In the majority of cases the sperm-duct or ducts open near or into a glandular structure which in earthworms has been called "prostate"; in the aquatic forms, on the other hand, "atrium." As these terms are objectionable from the different way in which they have been used for structures of Vertebrates, I have suggested for both the term "spermiducal glands," indicating the identity of the structure in all Oligochaeta. The number of pairs of these glands varies, as does also their shape and size. The typical form is perhaps illustrated in the lower Oligochaeta, where there is but a single pair into which the sperm-duct or ducts of the same side open. The Naids, Tubificidae, Lumbriculidae, and Moniligastridae have a simple gland of this description on each side of the body. These glands may consist of a tuft of pear-shaped glandular cells attached to the organ at one side, as in most Tubificidae, or of a complete investment of gland-cells, as in Branchiura. Among earthworms it is only the Moniligastridae and the Eudrilidae in which the sperm-duct opens directly into the end of the spermiducal gland; in the Perichaetidae the gland is differentiated into two sections; there is a muscular duct leading to the exterior, and a lobate glandular part, which is formed by a complicated branching of a single sac such as exists in the Tubificidae; in the Acanthodrilidae and in many Cryptodrilidae the spermiducal glands are of a tubular form and are not branched, though there is the same differentiation into a duct and a secreting portion. There are in the Acanthodrilidae two pairs of these, and as many as three pairs in Dichogaster; in the latter case in three successive segments. In the Acanthodrilidae the glands are upon the seventeenth and nineteenth segments. In most Cryptodrilidae the sperm-ducts do not open into the duct of the spermiducal gland, but on to the body-wall near to its orifice, the distance varying in different genera. In the Acanthodrilidae the male pore is on the eighteenth segment, removed therefore by the distance of a segment from the aperture of either of the glands. It may be that a large series of structures which exist in Microchaeta benhami[[416]] and in other Geoscolecids, and which have been termed copulatory glands, are the equivalents of the spermiducal glands.
Fig. 192.—Diagrammatic longitudinal section of Lumbricus, showing the generative segments. × 3. (After Hesse.) sp, Spermathecal pore; t, testis; s.s, seminal sac; sp.s, sperm-sac; o, ovary; e.s, egg-sac; ♀, female pore; ♂, male pore.
In many earthworms there are, at the external opening of the male ducts, bundles of specially modified chaetae, which have been called, from their supposed function, penial chaetae; they are usually ornamented at the free end with spinelets or ridges, and frequently offer valuable specific characters. In the Lumbricidae and the Geoscolicidae there are modified chaetae upon the clitellum; in a few forms, such as, for example, Acanthodrilus schmardae, the spermathecae have bundles of similar chaetae in their neighbourhood, often associated with glands not unlike the spermiducal glands.
In most, perhaps in all Oligochaeta the sperm is not matured in the testes, or even in the body-cavity; it is received into special sacs which are called sperm-sacs, and there ripens. These sacs, the vesiculae seminales, have been shown to be outgrowths of the septa; their cavity is thus a portion of the body-cavity shut off more or less completely from the general body-cavity.
The reproductive organs of the Eudrilidae, and particularly the female organs, are so divergent in many particulars from those of other Oligochaeta that it is convenient to treat them separately. The testes are normal, save that they are often adherent to the posterior wall of their segment, as, however, is the case with some other earthworms. In many Eudrilidae, for instance in the genus Hyperiodrilus, the funnels of the sperm-ducts are dependent from the anterior wall of the segment which contains them; the narrow tube which follows projects into the segment in front, and is there immediately dilated into a wide chamber, which again narrows, and bending round, re-traverses the same septum; the two ducts of each side (if there are two, which is not invariably the case) remain separate and open separately into the glandular part of the spermiducal gland. There is occasionally only a single median gland; and as a general rule the two glands open by a median unpaired orifice. Penial chaetae may or may not be present.
Fig. 193.—Female reproductive organs of Hyperiodrilus. XII-XV, Segments of the body; 1, spermathecal sac; 2, egg-sac; 3, spermatheca; 4, ovary.
The structure of the female organs differs considerably in detail in the different genera. But Hyperiodrilus may be taken as an instance of a genus in which these organs are as complicated as they are anywhere. The ovaries (Fig. 193, 4) are perfectly normal in structure and in position. So also are the oviducts; but both are enclosed in sacs which communicate in rather an elaborate fashion. Each ovisac is somewhat rounded in form, and the two communicate by a narrow tube; from the ovisac also arises another narrow tube, which soon dilates into a chamber lying in the thirteenth segment; this contains the mouth of the oviduct and is continuous with the egg-sac; the latter is quite normal in position. Beyond the egg-sacs the two tubes unite round the intestine and open into a large median sac, which contains sperm and may be called the spermathecal sac (1). There is, however, a true spermatheca, single and median. This opens on to the exterior in the middle of the thirteenth segment, but lies chiefly in the right-hand sac behind the ovarian portion of the same. I never found this spermatheca to contain sperm. Dr. Rosa inferred on anatomical grounds, and I have been able to prove developmentally (in Libyodrilus), that these sacs which involve the ovaries and oviducts, and which also contain sperm, are derivatives of the septa; that in fact the spaces which they enclose are coelomic. In some Eudrilids these sacs are the only "spermathecae"; in others, as in Hyperiodrilus, there are in addition blind pouches lying within them which must be regarded as true spermathecae; these are smaller in some than in others. In fact there are various transitions in the entire replacement of true spermathecae apparently homologous with those of other earthworms by pouches which are derived from the septa, and which are therefore of an entirely different morphological significance; here is an excellent case of the substitution of organs, analogous to the replacement of the primitive notochord of the Vertebrate by the vertebral column.
So far as is known, all the Oligochaeta deposit their eggs in special chitinous cases, the cocoons. They share this peculiarity with the Hirudinea. The cocoons have long been known, but were originally mistaken for the eggs themselves. The cocoons contain several eggs and a variable quantity of albumen for the nutrition of the growing embryos. In the majority of earthworms they are more or less oval with projections at the two ends, and are of a brownish colour. In others the tint is rather to be described as green. The genera Criodrilus and Sparganophilus have a cocoon which is greatly elongated. These structures seem to be undoubtedly formed by the clitellum, the earlier opinion of D'Udekem being that they were the product of certain glands developed in Lumbricus at the breeding season, which he thence called the capsulogenous glands. It is more probable that these glands, which have been up to the present but little investigated, are the seat of the formation of the albumen which is found within the cocoons. The cocoons are deposited at varying depths in the ground, or on the surface. Among the aquatic genera they are often attached to aquatic plants. The process of formation has been carefully watched by Vejdovsky[[417]] in the genus Rhynchelmis. The worm throws off the cocoon over its head, crawling backwards to free itself therefrom. The eggs, spermatozoa and albumen, reach the interior of the cocoon as it passes over the orifices of the respective ducts. Out of the numerous eggs which a single cocoon originally contains, only a few, sometimes only one, reaches to maturity. Among the Enchytraeidae, however, quite a large number of young emerge from a single cocoon. The development of all the Oligochaeta is direct, there being no free larval stage. It seems to be the rule for a process of fission to take place in the embryos of Allolobophora trapezoides[[418]] at least, according to the observations of Vejdovsky, in warm weather. In cold weather he found in each cocoon as a rule single embryos, and only 10 per cent of double embryos.