The gill-slits usually have the form of the so-called "branchial pouches" or "gill-sacs" (Figs. 5, 6, g.s). Each ordinarily opens to the exterior by a small pore (Fig. 1, D, 5, g.p) or slit, situated on the dorsal side, in a shallow longitudinal groove not far from the middle line. The gill-sac has a complete wall of its own, and lies between the alimentary canal and the body-wall, communicating with the former by a U-shaped slit. While a dorsal view of the animal thus shows a linear series of simple pores, a view of the pharynx from the inside appears as in Fig. 5.
At the hind end of the pharynx the inner opening of the developing gill-sac is circular. Slightly further forward the dorsal side of the pore is indented into a crescent, which grows longer in a dorso-ventral direction, and becomes a U, whose two limbs are nearly separated by a mass of tissue, the so-called "tongue-bar" (Fig. 5, t). The special interest of this mode of development is that it is identical with what occurs in Amphioxus (p. 120), which is universally admitted to belong to the Chordata.
The gill-sacs of Balanoglossus follow one another closely, the hind wall of one being in contact with the front wall of the next, and constituting a "branchial septum" (b.s). Both septa and tongue-bars are supported by chitinous rods, which are special thickenings of the membrane at the base of their epithelium. Two rods occur in each tongue-bar, separated by an interval of body-cavity (Figs. 5, 6), and only one rod in each septum. Originally of this form—∩∩ ∩∩—the rods have joined in pairs, the united limbs forming the single rod of each branchial septum. In this respect again we have a similarity between Balanoglossus and Amphioxus, except that in the latter the concrescence proceeds one step farther, and the two rods of the tongue-bar unite, like those of the branchial septum. The latter, the so-called "primary" skeletal rods of Amphioxus, are forked ventrally as in Balanoglossus (Fig. 5).
Fig. 5.—Diagram of two gill-sacs of Balanoglossus, seen from the inside of the pharynx. b, Branchial skeleton, consisting of a single forked bar in each branchial septum (b.s), and of two bars in each tongue-bar; g.p, gill-pore, opening on the dorsal surface of the trunk; g.s, gill-sac; s, synapticulum (only one or two shown); t, tongue-bar. The arrows indicate the communications of the gill-sacs with the exterior and with the pharynx.
In Amphioxus, as in most Enteropneusta, adjacent rods are connected at intervals by chitinous "synapticula" (Fig. 5, s), which traverse one or the other of the halves of the gill-slit. In Dolichoglossus, where no synapticula occur, the tongue-bars may be turned inside out by slight pressure, and then project to the exterior through the gill-pores.
The subdivision of the branchial region of the alimentary canal into two parts, as shown in Fig. 4, is characteristic of Glossobalanus and its allies. In Dolichoglossus and Glandiceps there is no such constriction, the region occupied by the gill-slits being merely the dorsal half of a tube with a simple circular section. Schizocardium (Fig. 6) agrees with Amphioxus in the fact that the gill-slits occupy nearly the whole of the wall of the pharynx; the only parts not perforated by gill-slits being the small dorsal and ventral portions.
In Ptychodera (Fig. 4), the gill-sacs are practically absent. The U-shaped slits of the pharyngeal wall thus open directly to the exterior,[[20]] and can be seen from the outside. In species which have this arrangement, the genital wings are greatly developed, so as to arch over the back of the branchial region. The gill-slits thus open into a kind of "atrium," resembling that of Amphioxus in its relation to the gill-slits, and in having the generative organs on its outer side, but differing from it in being dorsal to the pharynx.
Fig. 6.—Schizocardium brasiliense; transverse section through the branchial region, showing the great extent of the branchial part (b) of the pharynx; the oesophageal part (o) is reduced to a mere groove; g, gill-pore; g.s, gill-sac; r, reproductive organ; s, synapticula (cf. Fig. 5); t, tongue-bar. The muscles of the body-wall are not indicated: in other respects the figure corresponds with Fig. 4, except for the absence of genital wings in this region of the body. (After Spengel.)