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.
In the movement of the blood, then, in Nais as in Lumbricus, there are discernible only two leading directions - one forward in the primary and intestinal dorsal vessels, the other backward in the primary and intestinal ventral. It is not possible to trace the blood into the capillary parietal system of the intestine, in consequence of the transparency of the stream when thus minutely subdivided. In Nais there is also an integumentary system which intervenes between the two primary (dorsal and abdominal) trunks (a,f), ramifying in the substance of the integuments, upon which, in part, a respiratory function may devolve.
(632). The generative system of the Nais, as delineated by Duges, presents a very different arrangement to that which exists in the Earthworm. The swollen part of the body, in which the sexual organs are placed, occupies a space of five or six rings, beginning at the eleventh. On each side of the eleventh segment is a minute transverse slit (fig. 117, b) communicating with a slightly-flexuous canal which terminates in a transparent pyriform pouch or vesicle. The latter contains a clear fluid, wherein minute vermiform bodies are seen to float, and most probably represents the testis. The twelfth segment likewise exhibits two openings, each placed upon the centre of a little nipple (c): these arc the orifices leading to the female portions of the sexual system. The ovaria (d, e) are composed of four large and several smaller masses of a granular character; and from them proceed long and tortuous oviducts, which, just before their termination at the lateral openings (c), become thick and glandular. These animals most likely copulate like the Earthworm, and lay their eggs in a similar manner. We have already seen, in the Lumbricus terrestris, ova containing two yelks, and consequently giving birth to two animals; but in the Nats every egg produces ten or a dozen young ones *; or perhaps we ought rather to say that what appears to be a single egg is in fact merely a capsule enclosing several distinct ova, from which a numerous progeny arises. The manner in which these compound eggs are formed is easily understood when we consider the structure of the oviduct described above.
The granular germs escape, no doubt, separately from the ovaria, and remain distinct from each other as they pass along the tortuous canal that leads to the external opening; but at length arriving at the thick and glandular portion (c) of the ovi-gerous tube, several of them become enclosed in a common investment secreted by the walls of the oviduct, and are expelled from the body with the outward appearance of a simple egg.
Fig. 116. Plan of circulation in Nais. (After Dr. Williams).
The account of the reproductive system of the Naides given by Dr. Williams1 is as follows:
(633). In Nais serpentina, which may be taken as the type of the genus, two of the 'segmental organs' are present in every ring of the body, one on either side of the intestine. Each organ commences (or, if the course of the contained current be considered, ends) in a single external orifice. At its free internal extremity it is held in a determinate position by means of bridles of delicate threads. By the action of large and vigorous cilia, a strong current of fluid is drawn into this open mouth. From the form and structure of the trumpet-shaped extremity, as well as from the setting of the ciliary currents at the mouth and base of the tube, it seems beyond the possibility of a doubt that these organs, in this ordinary state, subserve the function of discharging externally the fluid contained in the general cavity of the body; nevertheless, from their small diameter, it would appear that they are not capable of conveying externally either the normal corpuscles of the chylaqueous fluid or the spermatic products.
The preceding description applies only to the 'segmental organs' distributed throughout that part of the body of the worm which is situated behind the reproductive mass.
Fig. 117. Generative organs of Nais. (After Duges).
* Duges, Ann. des Sci. Nat. vol. xv. 1 Phil. Trans. 1858.
(634). It now remains to investigate the claims of this organ to a new and hitherto unthought-of relation with the generative or reproductive structures; for in this worm two, or possibly more, of these segmental organs on either side of the body undergo a remarkable increase of size and variation of form.
(635). In Nais it often happens that only one on either side is thus evolved. At some seasons, however, especially when the weather has continued for some time warm and dry, individuals may be found in which two on each side have undergone a marked development; generally, however, two segmental organs only, the one being developed into the male, the other into the female moiety of the reproductive system, are necessary to the generative maturity of the individual.
(636). The two segmental organs which form the basis of the reproductive masses are observed, even on the first view, to be very similar in general outline to those which are repeated in every ring of the body, with which indeed they are identical, differing only in size and in the shape of some of their parts. The dilated portions correspond with the equivalent parts of the other organs; and the umbrella-like extremities, which are the same in form on the ovarian and testicular sides, are the counterpart of the free ciliated ends of the ordinary organs. The intermediate ciliated tubes are specially distinguished only by their greater length and thickness. It is a difficult point to settle the mode in which the generative masses are related to the ciliated tubes; it may, however, be constantly seen that the ovarian mass moves to and fro with the dilated portion of the tube. It is impossible to detect the opening by which the ova arrive in the interior of the utriculus of the tube; that they do get there, however, is certain.