Under the first, the chylaqueous fluid alone is submitted to this process; under the second, the blood-proper fulfils the office. The mechanical organs subservient to this function under the former are constructed on a plan diametrically different from that of those provided under the latter circumstances. In the Annelid, the true blood and chylaqueous fluid, though coexistent in the same organism, constitute two perfectly distinct and independent fluid-systems. There is between them no direct communication of any sort; they are, physically, very dissimilar fluids. An order of branchial organs must therefore be recognized in which, in equal or unequal proportions, the chylaqueous fluid and the blood-proper, either in the same or in distinct appendages, participate in the process of respiration." - Ann. & Mag. Nat. Hist. 1853, vol. xii. p. 393.

680 Tubicola 155

Fig. 132.

Terebella Medusa.

Fig. 133. Terebella Medusa.

(682). The branchial organs, in the genus Terebella*, appear under the form of blood-red tufts, proceeding from three separate root-vessels on either side of the occiput. The vessels divide for the most part dichotomously, forming an arborescent bunch of florid blood-vessels; each ramusculus is enclosed in a delicate cuticular envelope perfectly destitute of cilia, and is, moreover, double - that is, composed of an afferent and efferent vessel. Although extremely transparent and attenuated, the cuticular structure embracing these branchial bloodvessels must include some retractile fibres, since each separate ramusculus may be emptied and rendered bloodless by the compression of the parietes, a provision which frequently exists in many parts of the circulating system of the Annelida.

(683). The cephalic tentacles of the Terebellae present a problem interesting alike to the physiologist and the mechanician. Prom their extreme length and vast number, they expose an extensive aggregate surface to the agency of the surrounding medium. They consist, in Terebella nebulosa, of hollow, flattened, tubular filaments, furnished with strong muscular parietes. Each of these hollow band-like tentacula may be rolled longitudinally into a cylindrical form, so as to enclose a hollow semicircular space if the two edges of the band meet, or a semi-cylindrical space if they only imperfectly meet. This inimitable mechanism enables each filament to take up and firmly grasp, at any point of its length, a molecule of sand, or, if placed in a linear series, a row of molecules. But so perfect is the disposition of the muscular fibres at the extreme end of each filament, that it is gifted with the twofold power of acting on the sucking and on the muscular principle. When the tentacle is about to seize an object, the extremity is drawn in, in consequence of the sudden reflux of fluid in the hollow interior; by this movement a cup-shaped cavity is formed, in which the object is securely held by atmospheric pressure: this power, however, is immediately aided by the contraction of the circular muscular fibres.

Such are the marvellous instruments by which these peaceful worms construct their habitations, and probably sweep their vicinity for food.

(684). The inferior aspect of each of these tentacles is profusely clothed with cilia, and this side is thinner than the dorsal. The peritoneal fluid, which is so richly corpusculated, and which freely enters the hollow axes of all these tentacles, is thus brought into contact with the surrounding water.

* Dr. Williams, loc. cit. p. 194.

(685). In addition to the two important uses already assigned to the tentacles in the Terebellae, they constitute also the real agents of locomotion. They are first outstretched by the forcible ejection into them of the peritoneal fluid, a process which is accomplished by the undulatory contraction of the body from behind forwards; they are then fixed like so many slender cables to a distant surface; and then, shortening in their lengths, they haul forwards the helpless carcass of the worm.

(686). In the Terebellae, in consequence of the concentration of the tentacles and branchiae around the head, the blood-system at this extremity of the body discovers a great increase of development. The peritoneal fluid in this genus is very voluminous and densely corpusculated; the system of the blood-proper is, notwithstanding, elaborate and full-formed. The chamber of the peritoneum is one undivided space, - the segmental partitions of the Earthworm and the Leech being here replaced by limited bands proceeding from the intestine to the integument, tying together these two cylinders - so, however, as to permit one to move within the other with remarkable freedom.

(687). The great dorsal vessel in Terebella nebulosa is limited to the anterior of the body (fig. 134, a.) It emanates chiefly from a large circular vessel (b) embracing the oesophagus, and which receives all the blood from the intestinal system. In this species, therefore, the primary and intestinal dorsal trunks over the whole intestinal region are united, or the former vessel is superseded by the latter.

(688). On the dorsal view of the oesophagus, a large, pulsatile, fusiform vessel (a) is displayed on the first laying open of the integument in a longitudinal direction. Slightly attached to the structure on which it rests, it appears as if suspended in the fluid of the peritoneal cavity. Advancing to the occipital ring, it breaks out into six branches (d), of which three proceed to the branchiae of each side, while the reduced continuation of the original trunk furnishes minute ramuscules to the tentacles, in the hollow axes of each of which an afferent and efferent vessel is contained, surrounded by the peritoneal fluid, which penetrates to the remotest ends of these exquisite organs. Both from the tentacles and branchiae the blood now returns into the great ventral trunk (c), which to the posterior extremity of the body is distinct from, and independent of, the intestinal system (f.) From this trunk branches are detached on either side of the median line for the supply of the feet and integument.