Fig. 3.—Dorsal view of the anterior end of the body of Dolichoglossus kowalevskii, × 3. c, Collar; c.n, circular nerve; c.p, collar-pore; d, dorsal nerve; g, gill-pore; n, notochord; n.s, central nervous system, showing the anterior and posterior neuropores; p, proboscis; p.p, proboscis-pore; p.s, proboscis-stalk; t, trunk; v, ventral nerve. The nerve-plexus of the proboscis is represented as a black line. (After Bateson.)
The collar-cavities open by two "collar-pores" (Fig. 3, c.p.), situated at the posterior end of the collar, into the first pair of gill-pouches, near their external opening. Willey has recently described[[16]] vestigial pores in relation with the "perihaemal spaces," a pair of dorsally situated outgrowths of the third body-cavities into the collar-region. Narrow "peripharyngeal spaces," also a forward growth of the third body-cavities, closely invest the pharynx in some species.
Body-Wall and Nervous System.—The body-wall (Fig. 4) consists externally of a thick ciliated epidermis (e), containing numerous gland-cells which secrete an abundant mucus. Beneath the epidermis is a basement-membrane, while more internally are layers of muscles, whose arrangement differs in different parts of the body and in different species.
The nervous system consists of a plexus of cells and fibres which lie in the basal part of the epidermis of all parts of the animal, outside the basement-membrane; the thicker portions of the plexus forming definite nerve-tracts. This intimate connexion between the epidermis and the nervous system is usually restricted to embryonic life in other animals.
Fig. 4.—Ptychodera bahamensis, Bahama Is. Transverse section through the branchial region. b, Branchial part of pharynx; b.c3, third body-cavity; d.m, dorsal mesentery; d.n, dorsal nerve; d.v, dorsal vessel; e, epidermis, with nerve-layer (black) at its base; g, genital wing; g.p, gill-pore, encroached on by the tongue-bar (t); l, lateral septum; m, longitudinal muscles; o, oesophageal or alimentary part of pharynx; r, reproductive organ; t, tongue-bar; v, ventral mesentery and ventral vessel; v.n, ventral nerve. (After Spengel.)
The main nerves of Balanoglossus are a dorsal and a ventral tract in the trunk region (Fig. 4, d.n, v.n), a circular tract (Fig. 3, c.n) connecting these two at the posterior edge of the collar, and a strong concentration of nerve-tissue round the whole of the proboscis-stalk, and of the posterior end of the proboscis (Fig. 3). In the region of the collar the nervous system attains its highest development, taking the form of a median cord passing above the alimentary canal. This cord, known as the central nervous system (Fig. 7, n.s), runs through the cavity of the collar, but is connected with the epidermis at each end. It thus becomes continuous in front with the nerve-layer on the proboscis-stalk, while posteriorly it passes into the dorsal and the circular nerve-tracts. In nearly all cases the epidermis is pushed into the cord at the points where it passes into the skin, in the form of an anterior and a posterior "neuropore" (Fig. 3). A transverse section through the extreme front or hind end of the collar accordingly shows a tubular nervous system. In certain species, as in Glossobalanus sarniensis and Ptychodera flava, a central canal, opening in front and behind, exists throughout the entire length of the central nervous system, while in G. minutus a canal of this kind occurs in the young animal, but not in the adult. The central nervous system is developed as a longitudinal dorsal groove in the larva,[[17]] and in a similar manner in the collar which is formed as the result of regeneration after injury.[[18]] Balanoglossus is thus typically provided with a dorsal, tubular, central nervous system, and although this arrangement does not extend beyond the limits of the collar, it shows a noteworthy resemblance to Vertebrate animals.
In some cases the central nervous system is connected with the dorsal epidermis by a varying number (1-17) of median "roots," which have been compared by Bateson with the dorsal roots of the spinal nerves of Amphioxus, and are probably remains of the embryonic connexion of the collar nervous system with the dorsal epidermis.
Alimentary Canal.—The mouth (Fig. 7, m) leads widely into the alimentary canal, which, passing through the collar, enters the branchial region, where it is characterised by the existence of communications with the exterior. These, the gill-slits, are developed, as in Vertebrates, as paired outgrowths of the alimentary canal, and new gill-slits are constantly being formed at the posterior end of the branchial region with advancing age. The maximum number of the gill-slits, and the extent of the branchial region, are by no means uniform throughout the Enteropneusta. Thus Dolichoglossus otagoensis is said to have no more than 12 pairs, Glossobalanus minutus only 40 pairs, while Balanoglossus aurantiacus may have as many as 700 pairs. In Ptychodera flava the variation is so great that Willey distinguishes[[19]] two extreme conditions as "macrobranchiate" and "brachybranchiate" respectively, although intermediate conditions are also found. It should be noted that Balanoglossus agrees with Amphioxus in the indefinite number of the gill-slits.