At a certain distance behind the branchial region, the alimentary canal in Balanoglossus and Schizocardium is produced into a series of outgrowths, into which food does not pass. These "liver-sacs" give rise to corresponding folds (Fig. 1, A, h) of the dorsal body-wall, a conspicuous external feature of the species in which they are present. The most interesting peculiarity of the digestive tract in this region is the existence, in certain species, of pores, possibly vestigial gill-slits, leading from it to the exterior.
Notochord and Skeleton.—The structure compared by Bateson with the Vertebrate notochord is a hollow dorsal outgrowth of the alimentary canal of the collar-region (Fig. 7, n). Near its origin it is slender, but in the proboscis it dilates into a comparatively large organ, which in most cases retains its cavity. Its cells have a vacuolated appearance, which recalls the fine structure of the Vertebrate notochord. In Schizocardium and Glandiceps, the organ is produced into a slender "vermiform process" (v), which extends nearly to the tip of the proboscis.
Fig. 7.—Schizocardium brasiliense; longitudinal, median section through the proboscis, the collar, and the first part of the trunk; b, main blood-space of the proboscis; b.c1, b.c2, b.c3, first, second and third body-cavities; c.m, circular muscles of proboscis; e, epidermis; l.m, longitudinal muscles of proboscis; m, mouth; n, notochord; n.s, central nervous system, continuous with the subepidermic nerve-plexus (black) of the proboscis, and with the dorsal nerve (d); p.c, pericardium; p.s, proboscis-stalk; s, proboscis-skeleton; v, vermiform process of notochord. (After Spengel.)
The main support of the proboscis-stalk is the "proboscis-skeleton" (s), a Y-shaped organ whose median part lies beneath the base of the notochord, its diverging legs extending backwards along the outer side of the alimentary canal of the collar. The proboscis-skeleton, like the branchial skeleton, is a special development of the structureless membrane which is found at the base of the layers of cells of Balanoglossus, and in most species it grows merely by the deposition of laminae of chitin from the notochord, and from the ventral epidermis of the proboscis-stalk.
In some species, however, and particularly in Balanoglossus aurantiacus and Glandiceps, the primary skeleton becomes surrounded by an extensive development of a secondary cartilaginoid skeleton, consisting of a structureless substance into which the adjacent body-cavities of the proboscis and collar send cellular outgrowths. The possibility of a relation between this tissue, more or less surrounding a part of the notochord, and the cartilage of Vertebrates cannot be overlooked.
The caudal region may be stiffened (?) by a "pygochord"[[21]] which is a median derivative of the alimentary canal on its ventral side.
Vascular System and Proboscis-Gland.—The main vessels are a dorsal and a ventral vessel (Fig. 4, d.v, v), lying in their respective mesenteries. The details of the vascular system are complicated, and have not been thoroughly made out, the nearly colourless character of the blood making their investigation a difficult matter. The following points may, however, be noted. The blood is said to pass forwards in the dorsal vessel, which, like the ventral vessel and a pair of lateral vessels in the hepatic region, is contractile. In the collar the dorsal vessel lies between the two perihaemal spaces, on the dorsal side of the base of the notochord. The principal blood-space in the proboscis (Fig. 7, b) lies between the notochord (n) and an organ known as the "heart-vesicle" or "pericardium" (p.c). The latter has muscular walls and it contracts rhythmically in the larva. Its behaviour in the adult is not so easily made out, but it is probable that, although it does not communicate with the vascular system, its contractions propel the blood contained in the space immediately beneath it. The blood, after passing to a glandular organ, the "proboscis-gland" or "glomerulus," which lies at the sides and in front of the notochord, appears to pass round the collar to the ventral vessel. Various systems of vessels are connected with the skin, the gills, the alimentary canal and the generative organs.
The function of the proboscis-gland is possibly excretory. In this case it is probable that the proboscis-pore eliminates the waste products discharged by the gland into the anterior body-cavity, though this view is not favoured by Willey.
Reproductive Organs.—The sexes are separate, the reproductive organs consisting of a series of simple or branched glands which occur along the dorso-lateral lines of the anterior part of the body; being usually found throughout the branchial and generative regions and ending at the beginning of the hepatic region. The reproductive organs may pass into great extensions of the body-wall known as the "genital wings," specially developed in some species of Balanoglossus and Ptychodera (Figs. 1 A, 4).