(651). Although, as a whole, forming a cylinder, in no instance does the alimentary canal of the Annelida present the figure of a smooth-walled tube. The parietes are invariably sacculated, and often superficially multiplied in the most elaborate manner. In the Lumbriciform species, each segment of the body has its own independent stomach. Those of contiguous segments communicate through an opening considerably more contracted in diameter than that portion of the intestine from which it leads. Thus the intestine of the Errant Annelids, especially, may be compared to a line of pears, the apex of each successive pear being applied to the base of its predecessor in the series: if these bases were prolonged on each side, the stomach of the Leech would be the result; if compressed, that of those species in which the tube is nearly straight.

(652). The course of the principal trunks of the circulating system in the Dorsibranchiata bears a general resemblance to what we have already seen in the Abranchiate order, modified, of course, by the variable position of the branchial tufts. The annexed figure (122) of an elaborate dissection of an Amphinome (A. capillata), copied from one of the beautiful drawings contained in the Hunterian Collection*, affords an example of a circulating system in which the propulsion of the blood is effected entirely by vessels, without the intervention of any muscular cavities or heart. In this animal the respiratory organs are penniform appendages placed along the back, and these external vascular tufts communicate with delicate plexuses of vessels, situated in the interior of the body, called the branchial plexuses. In the figure the branchial plexuses of the left side only are represented (q q q); and of these, one marked q' has been turned aside. The blood and nutritious fluids derived from the whole alimentary track are collected by the large ventral intestinal vein (n n n) and conveyed to the branchial plexuses through the numerous vessels (o o o), some of which (o' o' o') are displaced in the drawing in order that their connexions may be better seen.

Besides the blood and nutriment thus derived from the intestine, the branchial plexuses receive the circulating fluid from all the segments of the muscular envelope by separate veins (p p); and thus the blood from all parts is brought to the gills and exposed to the influence of oxygen.

* Descriptive and Illustrated Catalogue of the Physiological Series of Comparative Anatomy in the Mus. Roy. Coll. Surg, of England, vol. ii. pi. 14.

(653). After undergoing respiration, the blood is collected from the branchial plexuses by the lateral veins (r r r), from which, through communicating vessels (s s s), it passes into the aorta or great dorsal vessel (t t t) to be distributed through the body. From the aorta large trunks (v v) are given off to form the intestinal artery (w w), which, ramifying over the intestine, communicates with the in-testinal vein (n n) and thus completes the vascular circle*.

(654). In Eunice sanguinea the circulatory apparatus consists of a short but capacious dorsal vessel (fig. 123, l'), which ovoid vesicle, or ampulla; it then runs outwards, furnishing an ascending branch to the alimentary canal, and on arriving at the base of the feet, or locomotive appendages, gives off several small anastomosing branches, forming a sort of vascular network, whence vessels are supplied to the corresponding branchial filaments.

Circulatory system of Amphinome capillata.

Fig. 122. Circulatory system of Amphinome capillata.

* The parts indicated in the drawing by letters not referred to in the text are the rests upon the pharyngeal portion of the digestive tube, without, however, being adherent thereunto, and communicates posteriorly with a vascular circle that surrounds the stomach and receives the blood from the intestinal parietes through two longitudinal trunks (fig. 123, I) situated upon the dorsal aspect of the alimentary tube.

(655). In its course towards the head, the single dorsal vessel (V), which is a continuation of the two dorso-intestinal veins (I), receives several branches, some derived from the digestive canal, others proceeding from the muscles and integuments of the neighbouring part of the back. These last-named branches communicate with a slender cutaneous me-dio-dorsal vessel that runs along the entire length of the body and receives from each segment numerous cutaneous ramusculi (x.) Lastly, the dorsal vessel gives off from its anterior extremity various branches to the cephalic segments, as well as others which are directed outwards, as in the Terebella described hereafter; but these, instead of supplying branchial organs, take a backward course, and are either distributed to the parts in the vicinity of the pharynx, or their ramifications anastomose with those of the ventral vessel, immediately to be described.

(656). The ventral vessel (fig. 123, q') gives origin, opposite each segment, to a pair of lateral branches; but the conformation of these branches, as well as the functions to which they are destined, are very different. Immediately after its origin, each lateral branch becomes considerably dilated and bends back suddenly upon itself, so as to resemble, when superficially examined, an following: - a a, the ventral surface of the segments of the body; e e, the ventral oars, or packets of bristles; ff, the ventral cirri, or feelers; g, the anal cirri; h, the anus; i i, k k, the bases of the dorsal and ventral oars, with their surrounding muscles; 11, the dorsal longitudinal muscular bands; m m, the ventral longitudinal muscular bands.

Circulatory and respiratory apparatus of Eunice sanguined.

Fig. 123. Circulatory and respiratory apparatus of Eunice sanguined: a, b, c, the antennae; e, the first segment of the body; f, lateral appendages, or rudimentary feet; ff, pharynx;g', mandibular muscles; i, intestine; l', vessel performing the functions of an aortic or systemic heart; I, superior intestinal vessels; s, their lateral branches (or branchial veins); q' ventral vessel; t, its lateral branches; t' contractile ampullae, performing the functions of branchial hearts; u, branchiae; x, subcutaneous vessels of the back. (After Milne-Edwards).