Marine Vermes with a ciliated epidermis, and a body divided into three regions, viz. a prae-oral contractile proboscis, a 'collar" surrounding the narrow base of the proboscis, and a long worm-like trunk. The first portion of this trunk is pierced by a double series of dor sally placed respiratory pores: the middle portion bears dorsal lobes formed either by prominent sexual glands, or sexual glands mingled with liver caeca; and the hind portion is distinctly ringed.
The mouth is ventral at the base of the proboscis in front of the collar; the anus terminal and widely open, being devoid of a sphincter. There is but one genus, Balanoglossus, with several species from different parts of the world.
The a?iimals exhale pectdiar and characteristic odours; are brightly coloured, the males differing in tint from the females; and live immersed in mud and sand, which they saturate with mucus. This mucus sometimes sets firmly, as in B. Robinii. Some species are phosphorescent.
The epidermis consists of a single (?) layer of cells, some with a delicate cuticle and cilia, others unicellular mucous glands 1. The body-cavity is divided into five sections, an anterior within the proboscis, two paired (right and left) in the collar, and two similarly paired in the trunk, derived respectively from as many outgrowths of the archenteron. The outer cellular walls of these outgrowths form the body-walls, composed of connective and non-striated muscular tissues. The latter is arranged as a well-developed longitudinal coat. In B. minutus there is also a delicate external circular coat, wanting in other species. The longitudinal coat consists of several concentric layers in the proboscis. The original cavity of the proboscis is filled by a network of stellate cells in the adult; their interspaces communicate with the exterior by a ciliated (epiblastic) canal opening at the base of the proboscis in the middle dorsal line, or on the left side (B. Kowalewskii), or by two such canals (B. Kupfferi). The inner walls of the archenteric outgrowths in the collar and trunk form the dorsal and ventral mesenteries, and the lateral transverse muscles of the digestive canal which are interrupted in the line of the mesenteries.
The dorsal mesentery is in some species absorbed from place to place in the adult. The cavity or cavities (right and left) in the collar may be much obliterated by growths of connective tissue, but communicate with the exterior bv a right and left ciliated (epiblastic") canal opening- into the first gill-slits.
1Bateson thinks that the second layer of epidermic cells, often supposed to exist, is due to the breaking off in teased preparations of the lower ends of the long cells, Q. J. M. xxvi. p. 513. or pouches. The cavities in the trunk are completely closed, but large. They contain a fluid, in which, when coagulated by reagents, Spengel observed stellate (? amoeboid) cells.
The nervous system consists of a dorsal and ventral cord, extending to the anus, and connected by a pair of ring-like thickenings in the collar, and of a delicate network of fibrils beneath the epidermis. The cords are sub-epidermic except in the region of the collar, where the dorsal cord is completely independent of the epidermis. The deep part of the cord is fibrous, the superficial part cellular. The dorsal cord in the collar contains a central lumen produced by invagination in B. Kowalewskii (Bateson), or either globular or long irregular cavities, lined by a cuticula (Spengel), found also in the middle and posterior regions of adults (Bateson). The dorsal cord is continuous at its anterior end, with a thick circular fibrous layer at the base of the proboscis, which thins away anteriorly. Ganglion cells occur at rare intervals in the cords, except in the dorsal cord of the collar, where they are numerous. Hence this part may be regarded as the central nerve-ganglion (Spengel). Organs of special sense are absent.
The digestive tract is straight. At its anterior end it sends forwards into the proboscis a diverticulum. The cavity of this process is obliterated anteriorly by cells which are vacuolated with nodal nuclei, thus resembling notochordal tissue (Bateson). On its ventral surface is formed a chitinous bar, prolonged behind into two rods, which lie in the walls of the digestive canal in the collar, and give attachment to longitudinal retractor muscles. The first section of the digestive tract is incompletely divided into a dorsal respiratory, and a ventral alimentary part, by two lateral longitudinal folds. The respiratory portion gives origin to paired gill-pouches. These pouches open externally on each side by small pores situated in a longitudinal dorso-lateral furrow: internally, by long U-shaped slits. New gill-pouches appear to be constantly added throughout life at the hind end of the series. They are at first circular, like the gill-pouches of the embryo; but attain the characteristic U-shape by the downgrowth of a dorsal process or valve. The sides of the pouches, and the partitions between them, are supported by chitinous lamellae. Their margins are ciliated, and water passes through them from within outwards. They receive their blood from the median dorsal vessel.
The section of the digestive tract behind the branchial region is traversed by a longitudinal dorsal and ventral furrow richly ciliated. In some species (B. mimitus, B. claviger, &c), the middle section of the tract possesses paired dorsal liver-caeca, the cells of which contain a brown or green pigment. These caeca pass outwards into lobes of the body-wall, and possess an external aperture in B. Salmoneus. For the dorsal a'nd ventral mesentery see supra. The latter is lost in the collar; as also is the dorsal mesentery, except in B. Kowaleivskii and B. Salmoneus.
The vascular system consists of a dorsal and a ventral vessel lying in the mesenteries, and connected in the collar by an oblique vessel on each side. The dorsal vessel enters the proboscis and ends in a sac, with muscular walls lying immediately above the outgrowth from the digestive canal. This sac is pulsatile, at least in the larva, and may be termed 'heart.' There is a system of blood sinuses beneath the epidermis and in the walls of the digestive tract. The blood contains no corpuscles. It is said that the current runs forwards in the dorsal, backwards in the ventral vessel. A peculiar structure lies above, in front of, and at the sides of the heart in the proboscis. It forms the proboscis gland of Bateson, the internal gill of Spengel. Its posterior part is saccular, while its anterior and lateral parts consist of a network of blood-vessels covered with cells. As these cells contain brownish granules, found also outside them in the proboscis cavity, the organ is probably excretory (Bateson).