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.
This dilated muscular portion of the segmental organ is not ciliated internally, but its walls are capable of contracting vermicularly or peristaltically.
(616). The vascular system in connexion with this apparatus is at certain seasons extraordinarily developed. When present under the most evident circumstances, it may be described as consisting of three parts. First, of two or three large vessels, which curve upwards from the ventral to the dorsal trunks. These vessels thus form a framework by which the slender ciliated tube is held vertically in situ, they themselves being vertically disposed.
(617). Between the primary vertical trunks extend a vast multitude of secondary branches, which further subdivide to form a dense plexus of smaller vessels, in the meshes of which the ciliated tube is entangled. At certain seasons and conditions of growth, a dense mass of florid blood is attracted to and retained in the region of the segmental organs. It is contained at these times, not in ordinary cylindrical vessels, but in capacious lateral caecal branches, terminating in large, bulbous, pear-shaped extremities, evidently constituting a special provision, the nature of which still remains a matter of conjecture.
(618). The sexual system of the Earthworm we have already found to be centred in a particular region of the body, indicated by the thickened glandular collar above alluded to, which, however, on close examination, is found to be situated several segments posterior to the generative masses contained within.
(619). Now, in studying the visceral contents of each ring within the limits of the generative region, it will be best to proceed from behind forwards. The dissector thus comes first upon the largest and most prominent of all the generative masses. They are testes, as may be proved by a microscopic examination of their contents. They have a white, glittering, oily colour. In figure they are intestiniform, the coils, of which there are two or three, being tied together by a kind of mesentery and enclosed in a membranous capsule. These two testicular masses, which lie across and fill up several rings, may be traced with perfect clearness to comparatively narrow peduncles, which, when minutely and successfully dissected, will be found to connect themselves intimately with the roots of the ciliated tubes or special segmental organs of the same annular spaces; or, in other words, the tube and the peduncle of the testicular mass have a common outlet. This outlet cannot be detected on the external surface of the worm; it is far too minute and closely contracted; but its position may be clearly ascertained by the termination, internally, of the ciliated tube, proving, according to the views of Dr. Williams, that the generative gland is an outgrowth from, and organically the development of the ' segmental organ.' The two ciliated tubes, one on either side, which are contained in this testicular ring, differ in no respect from those of the non-generative rings but in that of size.
They are considerably larger than the latter.
(620). The second generative annulus in the order mentioned is exclusively ovarian, and it is not difficult to see that the mass and the base of the ciliated tube run together, and become blended into one structure. The most minute dissection fails to isolate the duct which, it may be supposed, leads from the ovary; but, as in the case of the testes behind, it is certain that the gland and the tube must have a common outlet. The ovary is considerably smaller than the testes; its capsule is more dense and vascular, and its interior structure is much more copiously supplied with blood.
(621). In the common Earthworm the second, third, and fourth generative segments are ovarian, each being anatomically only a repetition of the other; all are constructed upon the same plan.
(622). The fifth segment, from behind, is again testicular, exactly resembling the first, so that the first and the last segments in this region are testicular, the three intermediate ones being ovarian.
(623). The ovaria of Lumbricus are much more transient in their duration than the testes; the latter, in a certain condition, are always present at every season of the year, the former only in the summer months. The ova, while yet in the ovaria, are beautifully clear transparent cells. In August and September they seem to consist of nothing but germinal vesicles; afterwards appear the germinal spots, and then the rudimentary vitellus. At a subsequent stage, just before their extrusion from the body, they become covered with a cocoon or characteristic capsule, each capsule containing many ova. This capsule is a compound of chalk and mucus. In the median line between the ovaria there are situated two or more glandular bodies, the contents of which, under the microscope, are found to consist of nothing but carbonate of lime - doubtless the source of the chalk.
(624). Both in the ovarian and testicular segments there are sacculi attached to the bases of the segmental organs, which in the former case serve as receptacles for the ova (vitellaria), and in the latter as receptacles for the semen. In one case the ova acquire their calcareous capsules; in the other the sperm-cells become developed into active spermatozoa.
(625). The eggs, when laid, are said by Duges to be two or three lines in length. In fig. 115, a, one of them, enclosing a mature embryo, is delineated; its top is seen to be closed by a peculiar valve-like structure adapted to facilitate the escape of the worm, and opening (fig. 115, b) to permit its ogress. Another remarkable circumstance observable in these eggs is that they very generally contain double yelks, and consequently two germs; so that a couple of young ones are produced from each.
(626). It is very generally believed that the Earthworm may be multiplied by mechanical section, the separated portions reproducing such parts as are removed in the experiment, and again becoming perfect.