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 half of the divided worm, like the former, gradually presents evidences of decay: it becomes less and less irritable, the muscles and integuments begin to decompose, the blood-vessels of the branchiae become black, and the whole disappears by the dissolution of the structures.
* Comptes Rendus, xvi. 1843. 1 Muller's Archiv, 1840.
(674). The Aphroditaceae:, or Sea-mice, are remarkable for the long hairy tufts with which their pedal appendages are generally furnished (fig. 129.) Nothing can exceed the splendour of the colours that ornament some of these fasciculi of hairs; they yield, indeed, in no respect to the most gorgeous tints of tropical birds, or to the brilliant decorations of insects: green, yellow, and orange - blue, purple, and scarlet - all the hues of Iris play upon them with the changing light, and shine with a metallic effulgence only comparable to that which adorns the breast of the humming-bird. But it is not for their dazzling beauty merely that these setae are remarkable; they are not un-frequently important weapons of defence, and exhibit a complexity of structure far beyond anything to be met with in the hair of higher animals. In the Aphrodite aculeata, for example (fig. 129, a), they are perfect harpoons; the point of each being provided with a double series of strong barbs (fig. 129, b); so that when the creature erects its bristles - much more formidable than those of the Porcupine - the most determined enemy would scarcely venture to attack it.
(675). But here we cannot help observing an additional provision, rendered necessary by the construction of these lance-like spines. We have before noticed that the bundles of setae are all retractile, and can be drawn into the body by the muscular tube from whence they spring. It would be superfluous to point out to the reader the danger which would accrue to the animal itself by the presence of such instruments imbedded in its own flesh, as by every movement of the body they would be inextricably forced into the surrounding tissues. The contrivance to obviate such an accident is as beautiful as it is simple. Every barbed spine is furnished with a smooth horny sheath (fig. 129, c, a, b), composed of two blades, between which it is lodged; and these, closing upon the barbs when they are drawn inwards, effectually protect the neighbouring soft parts from laceration.
Fig. 129. Aphrodite aculeata.
(676). In the Aphrodite we have an additional appendage developed from the upper part of each lateral oar, in the shape of a broad membranous scale, which, arching inwards over the back (fig. 130, c), forms with its fellows a series of imbricated plates, or elytra, as they are technically named (fig. 129, a.) Each of the elytral scales is formed by a double membrane, between the laminae of which at certain seasons the eggs are found to be deposited, - a situation evidently adapted to ensure the exposure of the ova to the influence of the surrounding element, and thus to provide for the respiration of the embryo.
(677). The Aphroditaceae, indeed, constitute a group of Annelids to which the term 'dorsibranchiate' by no means correctly applies; that is, in the majority of species embraced in this order, no branchial appendages exist, either on the dorsum or any other part of the body. Respiration is performed on a novel principle, of which no illustration occurs in any other family of worms. In all the Aphroditaceae the blood is colourless. The blood-system is in abeyance, while that of the chyl-aqueous is exaggerated. Although less charged with organic elements than that of other orders, the fluid of the peritoneal cavity in this famiy is unquestionably the exclusive medium through which oxygen is absorbed. The true Aphrodite type of respiration occurs in Aphrodite aculeata (fig. 129, a.) In this species, the tale of the real uses of the elytra or scales is plainly told. Supplied with a complex apparatus of muscles, they exhibit periodical movements of elevation and depression. Overspread by a coating of felt, readily permeable to the water, the space beneath the scales during their elevation becomes filled with a large volume of filtered water, which during the descent of the scales is forcibly emitted at the posterior end of the body. It is important to remark that the current thus established laves only the exterior of the dorsal region of the body. It nowhere enters the internal cavities; the latter are everywhere shut out by a membranous partition from that spacious exterior enclosure bounded above by the felt and elytra. The complex and labyrinthic appendages of the appendiculated stomach lie floating in this fluid and in the chambers which divide the roots of the feet. From this relation of contact between the peritoneal fluid and the digestive caeca, which are always filled by a dark-green chyle, it is impossible to resist the conclusion that the contained fluid is really a reservoir wherein the oxygen of the external respiratory current becomes accumulated.
From the peritoneal fluid the aerating element extends in the direction of the caeca, and imparts to their contents a higher degree of organization. These contents, thus prepared by a sojourn in the caeca of the stomach, become the direct pabulum for replenishing the true blood which is distributed in vessels over the parietes of these chylous repositories. The sequence of events now indicated will convey to the mind of the physiologist a clear idea of the mechanism of the processes both of respiration and sanguification, by an arrangement strikingly analogous with what we have already seen in the Asteridae amongst Echinoderms.
Fig. 130. Segment of Aphrodite.
(678). In order to arrive at a correct knowledge of the reproductive system of this Annelid, specimens of both sexes should be examined in the spring, and again in the autumn. A good example of a female Aphrodite being obtained, the dissection should be thus proceeded with: - Pin the animal down to the trough with its back upwards. Open it by a longitudinal incision extending from the tail to the head. The incision should cut through the scales, felt, and integuments, in order to lay open the spacious perigastric chamber. The integuments should now be carefully stretched, and pinned down to the sides, so as to expose the interior. Let the dissection be then gently floated in salt water, and the parts will present the appearance here described. A network of minute tubes or threads will be seen to twine round and embrace the diverticula of the alimentary canal.