This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(609). According to M. Duges1, the arrangement of the sexual parts is represented in the diagram, fig. 114. The testicles (b) are placed in successive segments of the body from the seventh backwards: they vary in number in different individuals, from two to seven; but whether this variety depends upon a difference of species, or is only caused by the posterior pairs becoming atrophied when not in use, is undetermined.
Fig. 113. Plan of the circulation in an Earthworm. (After Dr. Williams).
* Lectures on Comp. Anat. vol. iii. 1 Ann. des Sci. Nat. vol. xv.
Each testis is fixed to the bottom of the ring in which it is placed by a short tubular pedicle that opens externally by a very minute pore, through which a milky fluid can be squeezed. The testicular vesicles of the same side of the body all communicate by a common canal; and the contained fluid, which, like the seminal secretion of other animals, contains spermatozoids, can readily be made to pass from one to another.
(610). The ovaria (c) are eight large white masses, of a granular texture, from which arise two delicate tubes or oviducts; these have no connexion with the testes, but, running backwards, they become dilated into two small vesicles at their termination (d), and open by two apertures or vulvae seen externally upon the sixteenth segment of the body: in these ducts, eggs have been detected as large as pins' heads.
The following is Dr. Williams's account of this mysterious apparatus*: -
(611). The generative system of the Earthworm is situated in the immediate vicinity of a thickened ring or band, bounded by abrupt limits, which implicates six or eight of the annuli of the body. This thickening, when closely examined, is found to depend upon an extraordinary development of the cutaneous follicles. On the abdominal aspect of this thickened portion suctorial cups are formed, by aid of which, during the congress of two individuals, mutual contact is maintained; but the generative segments internally have no relation with this suctorial ring of integument, nor has this latter part anything to do with the true genitalia. It is, like the thumb of the Erog, a mere provision for the mutual apposition of two individuals. The enlarged follicles of this cutaneous ring, moreover, discharge another function - they supply the peculiar glutinous secretion which affords a protecting capsule to the ova as they escape from the body.
(612). Every ring in the body of this worm (except a few at the head and tail) contains two 'segment organs,' one on either side of the intestine; they are convoluted and tubular, arising from the ventral aspect of the general cavity near the median line, curving upward around the intestine, and terminating in a fan-shaped ciliated extremity, which is bridled to the septum near its dorsalmost edge. Those segmental organs which are situated anterior to the gizzard are very much larger and more distinct than those that are placed behind it.
Fig. 114. Arrangement of the sexual organs in an Earthworm. (After Duges).
* Phil. Trans. 1858.
(613). The season of the year and the state of the weather have much to do with the condition in which these organs are found. All the specimens upon which the following examinations were instituted were taken in the months of July and August, from a rich, loamy, highly cultivated garden soil. This fact it is material to know, since nowhere in the ordinary fields and meadows does this worm attain the same size and plumpness. The generative nisus does not seem to reach its climax until the worm has arrived at a certain period of age and fulness of growth, so that, out of a hundred specimens, only ten or fifteen may be found in that condition which is required for the successful prosecution of these researches.
(614). In Lumbricus terrestris, each ring of the body is divided from the adjacent ones by membranous partitions, which either completely or partially isolate the fluid contents of each annular space. It is probable, however, that the fluid of the general cavity freely oscillates from one extremity of the body to the other, through perforations in the septa. But whether the spaces be isolated or not, the 'segmental organs' which they contain have no connexion whatever with one another. The following account is descriptive of all those which are situated posteriorly to the proventriculus and the generative region.
(615). The tube which connects the free extremity with the fixed end is extremely convoluted, and thickly intermixed with vessels. This intermediate tube is divisible into three distinct portions. First, a smooth-walled membranous part, which extends from the umbrellalike termination to the camerated or cellular portion, which is vigorously ciliated in its interior. The ciliary current sets from the free end, in the direction of the fixed end. The next division of the tube extends from the termination of the smooth membranous part to the commencement of the third stage: this portion is not ciliated; the walls bulge out into lateral cells. The segmental organ in this portion is inextricably blended and matted up with blood-vessels. The fourth division of the tube stretches from the end of the celled portion to the commencement of the muscular part: this again is strongly ciliated; its walls are thick and contractile. The last, outermost, or dilated portion, bounded externally by the attached end, is almost invariably, in the months of July, August and September, crowded with the ova and young of a Nematoid Entozoon, which might easily be mistaken for the ova and young of the Earthworm, - an error into which Dr. Williams himself fell during his earlier researches.