They are worm-like animals which live an obscure life under stones and burrow in the sand by inflation of the collar and proboscis, progression is effected with marvellous rapidity. Like earthworm it burrows in the sand, which is ingested and is passed through the intestine, the exhausted sand being finally ejected through anus at the orifice of the burrow. The mouth is kept permanently open and prevented from collapsing by a pair of skeletal cornua belonging to a sustentacular apparatus.
When first discovered by J. F. Eschscholtz at the Marshall Islands in 1825, Balanoglossus was described as a worm-like animal belonging to the Echinoderm order of Holothurians or sea-cucumbers. In 1865 Kowalevsky discovered that the organs of respiration consist of numerous pairs of gill-slits leading from the digestive canal through the thickness of the body-wall to the exterior. On this account the animal was subsequently placed by Gegenbaur in a special class of Vermes, the Enteropneusta. In 1883-1886 Bateson showed by his embryological researches that the Enteropneusta exhibit chordate affinities in respect of the coelomic, skeletal and nervous systems as well as in regard to the respiratory system, and, further, that the gill-slits are formed upon a plan similar to that of the gill-slits of Amphioxus, being subdivided by tongue-bars which depend from the dorsal borders of the slits.
The coelom is divided into three portions,separated from one another by septa, namely, proboscis-coelom, the collar-coelom and the trunk coelom. Of these the first two communicate with the exterior by means of a pair of ciliated pore-canals placed at the posterior end of their respective segments. The collar-pores are remarkable for their constancy. Water renders the collar turgid during progression.
At the point where the stomochord opens into the buccal cavity, the nuchal skeleton bifurcates, and the two cornua pass obliquely backwards and downwards embedded in the wall of the proboscis.
The nervous system is thus essentially epidermal in position and diffused, an interesting concentration of nerve-cells and fibres. The collar nerve tube has a cavity. Special thickenings of the diffused nervous layer of epidermis occurs in certain regions of body. In the neck of the proboscis the fibrous layer is greatly thickened. From the ventral surface of the cellar nerve-tube numerous motor fibres may be seen passing to the subjacent musculature.
In most species of Balanoglossus each gill-slit may be said to open into its own atrial chamber or gill-pouch; this in its turn opens to the exterior. There are, therefore, as many gill-pouches as there are gill-slits and as many gill-pores as pouches. The gill-pores occur on each side of the dorsal aspect of the worm in a series at the base of a shallow groove, the branchial groove . The respiratory current of water is therefore conducted to the exterior by different means from that adopted by Amphioxus.
Excretion is mainly done by a structure peculiar to Enteropneusta called the glomerulus, a vascular complex placed on either side of the anterior portion of the stomochord, projecting into the proboscis coelom.
The vascular system itself is quite peculiar, consisting of lacunae and channels destitute of endothelium, situated within the thickness of the basement-membrane of the body-wall, of the gut-wall and of the mesenteries. Blood, which is a non-corpuscular fluid, is propelled forwards by the contractile dorsal vessel and collected into the central blood-sinus; this lies over the stomochord, and is surrounded on three sides by a closed vesicle, with contractile walls, called the pericardium. By the pulsation of the pericardial vesicle, the blood is driven into the glomerulus and then to the ventral vessel in the trunk .
Female gonads consist of more or less lobulated hollow sacs connected with the epidermis by short ducts in the pharyngeal region. The pharyngeal folds are called the genital pleurae because they contain the bulk of the gonads . Correlated with the presence of the genital pleurae there is a pair of vascular folds of the basement membrane proceeding from the dorsal wall of the gut in the postbranchial portion of the branchio-genital region. The development of Balanoglossus takes place according to two different schemes, known as direct and indirect by two kinds of ova, large and small . In direct development, adult form is achieved without striking metamorphosis. Two families of Enteropneusta, Ptychoderidae and Spengelidae, contain species of which all pursue an indirect course of development, culminating in metamorphosis by which the adult is formed. In these cases the larva, called Tornaria, is pelagic and transparent, and possesses a complicated ciliated band.