(670). In Arenicola piscatorum* the generative apparatus consists of six lateral pouches, which during the months of July and August are in a condition of extreme vascularity. They are quite visible to the naked eye by their bright red colour. The minute structure of these pouches can be studied by carefully dissecting them from their attachments to the abdominal wall of the cavity and placing them under the microscope. One of them thus viewed will be seen to consist of a sac or bag, the fundus of which is caecal. The interior cavity throughout the upper three-fourths is a single undivided space. The attached extremity is formed into two very distinctly marked Channels or tubes, the interior of which is formed of cilia that beat in opposite directions. In the glandular tube, the ciliary current sets towards the cavity of the organ; in the simple tube it sets out from this cavity. In the fundus the current leads up on one side, and down towards the outgoing limb on the other. The two limbs by which the organ is tied to the walls of the perigastric chamber are not similarly formed. The ingoing limb exhibits a more glandular character, and its walls are considerably thicker and more richly supplied with blood, than the other.

About the middle of its course it enlarges into a round gland-like body, the axis of which is perforated by the tube. The ingoing limb commences in an external orifice on the abdominal surface of the animal; so that the ingoing current which it serves to convey can only consist of sea-water, which being thus introduced into the cavity, is driven out again, in whole or in great part, by outwardly-acting cilia through the limb, which also opens externally: none penetrates into the cavity of the body. Nevertheless Dr. Williams has no doubt, although he has never been able to demonstrate the fact, that the outgoing leg of the looped organ not only communicates directly with the exterior, but also, by a lateral process, opens into the cavity of the body; and by means of this last process, the ova in the female and the sperm-cells in the male reach the perigastric chamber. The ova in the female and the sperm-cells in the male are abundantly found in the fluid of the general cavity; and the ova may also be actually seen in large crowds in the outgoing limb. The ova at this point consist of clear, pellucid germinal vesicles: the vitellus has not yet appeared; after they have sojourned for some time in the general cavity, the latter begins to show itself.

According to Dr. Williams, then, the segmental organ is the true ovary in the female and the true testis in the male. The ingoing limb of the organ is a highly glandular structure: its vessels are densely packed and specially arranged; its walls are thick and stromatous; and at its mid-point is a notable glandular development. From the vascular system of this gland proceeds the great vascular organ.

Circulation in Arenicola.

Fig. 128. Circulation in Arenicola.

* Dr. Williams, Phil. Trans. 1858, p. 118.

(671). To the one side of this great vascular system there are appended peculiar caecal pouches, from the other a dense capillary plexus. This vascular appendage is the morphological equivalent of the blood-system connected with the ovogenetic limb of the segmental organ of the Leech, and of the botryoidal apparatus of vessels connected with the segmental organ of Lumbricus, and must either be the receptacle of an extra supply of blood to an organ susceptible of periodical expansion, or it excretes something from the blood-proper into the cavity of the segmental organ which is essential to the further development of the generative products. It is quite certain that the ova and sperm-cells pass through the last stage of their development in the perigastric chamber. How they escape out of this chamber has never yet been proved.

(672). Rathke and Grube have argued that Arenicola is androgynous. De Quatrefages, however, from his knowledge of the development of the spermatic particles, has long recognized the existence of separate sexes*; and long before this, Stannius had concluded that the sexes were separate, from the fact that in different individuals the contents of the general cavity of the body were different 1. Stannius also observes that the parent sperm-cells leave the segmental organs (his testes) before the formation of the spermatozoa, which are found only in the cavity of the body. Krohn, Frey, and Leuckart assert that the ova and spermatozoa are developed free in the general cavity of the body.

(673). In relation to the spontaneous division of the Annelida, Dr. Williams observes: - It is true that, towards the latter end of every summer, certain species of worms are multiplied by a cutting across the body at one or more points. If the fissure occurs at more than one point, the animal, of course, becomes divided into more than two pieces. This circumstance seldom happens. The fissure in Arenicola generally occurs somewhere within the middle third of the body, securing a few branchial tufts for each fragment. The tail, however, is sometimes detached, and sometimes the division happens very near the head. This process, in Arenicola, happens during July and August. The cephalic and caudal pieces continue for some time to writhe in the sand, somewhat further down in the soil from the surface than the perfect individuals. Towards September, the fragments (both that attached to the head and that belonging to the tail) dissolve away, ring by ring, and finally disappear by decomposition. If the fragment examined be that of the tail, it will be observed at the point of separation to exhibit an eversion of the edges, placing the alimentary canal exteriorly; and a very evident increase of size in the vessels also occurs, accompanied by a tumefied state of all the structures of the part.

From this latter fact, it is easy to be misled into the idea that the vessels can become enlarged for no other purpose than that of repairing the injury done by the fissure, or, perchance, of reproducing the part detached by that process. Such would naturally be the meaning which a physiologist would attach to the swollen appearance of the blood-vessels. But such is not the conclusion to which the careful practical observer is conducted by the study of the actual phenomena of the process. It is, of course, indisputable that nature accomplishes some adequate object by the fissure of the body of the worm; but that object, whatever else it be, is unquestionably not that of multiplying the species. The tail-fragment never, as can be proved by easy observation, produces a single new ring or segment of the body. If this be true, how completely improbable must be the statement that the headless piece is capable of producing a new head! In Arenicola, Dr. Williams can confidently declare that such reproductive properties as those implied in the re-formation, and that too by a remnant of an integral part of the body, do not exist. It is equally inaccurate to maintain that a new tail is formed by the cephalic fragment.