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 {16}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).