Balanoglossus, the largest genus now recognised by Spengel, appears to be practically world-wide in its distribution; Schizocardium is recorded from both sides of S. America; Glandiceps from the Atlantic, the Mediterranean, Japan, and the Indian Ocean; Spengelia from the South Pacific; and other species from the White Sea to New Zealand. The habitat is usually sand or gravelly sand, in which the animal forms a kind of tube by means of the abundant mucus secreted by its skin. Dolichoglossus kowalevskii (Fig. 1, D), according to Bateson,[[9]] lives between tide-marks at a depth of about eight inches. The greater part of the body is coiled in an even, cork-screw-like spiral, while the anterior end, including the front part of the branchial region, is maintained in a vertical position. The posterior end is also kept upright, and can be moved up and down in a vertical shaft opening on the surface, thus enabling the animal to eject the undigested sand from its anus.
The coloration of Balanoglossus is often brilliant. That of D. kowalevskii[[10]] is as follows:—The "proboscis" (cf. Fig. 1, B, p) is yellowish white; the "collar" (c) is brilliant red-orange (especially in males), with a white ring posteriorly; the "trunk" (t), the subdivision of which into "branchial," "genital," "hepatic," "abdominal," and "caudal" regions is better indicated in other species (Fig. 1, A, b, g, h, ab), is orange-yellow, shading to green-yellow in the semi-transparent caudal region. The genital region is grey in females and yellow in males, a sexual difference in colour being common in Enteropneusta. The hepatic papillae of other species may be bright green.
The odour of D. kowalevskii resembles that of "chloride of lime with a faecal admixture," while that of Balanoglossus aurantiacus suggests iodoform. All Enteropneusta are said to have a more or less offensive smell. A species of Balanoglossus is known to be intensely phosphorescent.[[11]]
The mouth (Fig. 7, m) is situated on the ventral side, at the base of the proboscis, and is concealed by the free anterior edge of the collar, which encircles the thin "proboscis-stalk" (Fig. 3, p.s). The animal has the singular peculiarity of being unable to close its mouth;[[12]] and thus, as it burrows through the ground, the sand which passes into the alimentary canal leaves it in a continuous column through the terminal anus.[[13]] The large coiled "castings" formed in this way between tide-marks enable the experienced collector to infer the presence of Balanoglossus; and in a West Indian species described by Willey[[14]] they are so large as to form "an important feature in the landscape at low tide."
The principal agents in burrowing are the proboscis and collar. An animal observed by Spengel pushed the tip of its proboscis into the sand, waves of muscular contraction meanwhile passing over the surface of the proboscis. At first the animal made slow progress; but the collar, becoming surrounded by sand, soon became a point of resistance by means of which the proboscis could bury itself yet more deeply. The animal quickly disappeared as soon as the first two regions of its body were engaged in the task of burrowing[[15]]
This action is due partly to the muscles of the body-wall, but largely to the power possessed by the proboscis and collar of becoming swollen and turgid. Spengel has observed that these parts become flaccid when the animal is taken out of water, and can only swell again when it is replaced therein; and it may thus fairly be concluded that the enlargement is due to the taking in of water. This is probably in fact the most important function of the "proboscis-pore" and of the "collar-pores" which are described below.
Fig. 2.—Diagram of a dorsal view of a Balanoglossus-embryo, after the formation of the body-cavities, a, Alimentary canal; b.c1, body-cavity of the proboscis; b.c2, of the collar; b.c3, of the trunk. (From Bateson.)
Body-Cavities.—The existence of five separate body-cavities (Fig. 2) is one of the most fundamental facts in the anatomy of Balanoglossus. The first body-cavity, or cavity of the proboscis (b.c1), is single and unpaired; the second body-cavities (b.c2) are paired spaces, one belonging to each side of the collar; the third body-cavities (b.c3) are similarly paired, and correspond with the trunk. While there is no connection between successive body-cavities, there are in certain regions communications between the two cavities of the same pair. Each of the paired cavities is at one time a closed lateral space between the skin and the alimentary canal. As the two spaces which constitute the pair grow towards one another, both above and below the alimentary canal, they come into such close apposition that they remain separated only by their conjoined walls. In this way are formed the dorsal and ventral mesenteries (Fig. 4, d.m, v), the former being the only one to persist in the higher Vertebrates. The body-cavities of the adult become to a large extent disguised by being traversed by connective tissue and muscles.
The hinder part of the proboscis-cavity is divided by the forward growth of the notochord (Fig. 3, n) into dorsal and ventral portions. The dorsal cavity in extending backwards becomes further subdivided into right and left halves, the latter typically opening dorsally to the exterior on the proboscis-stalk by an asymmetrical "proboscis-pore" (p.p.). Two symmetrical proboscis-pores may, however, occur, or a median pore connected with the left division of the proboscis-cavity. These may be individual variations within the limits of a single species, or may occur as a normal feature of a species.