certain regions and along certain lines. In the neck of the proboscis the
fibrous layer is greatly thickened, and other intensifications of this
layer occur in the dorsal and ventral middle lines of the trunk extending
to the posterior end of the body. The dorsal epidermal nerve-tract is
continued in front into the ventral wall of the collar nerve-tube, and at
the point of junction there is a circular commissural thickening following
the posterior rim of the collar and affording a special connexion between
the dorsal and ventral nerve-tracts. From the ventral surface of the collar
nerve-tube numerous motor fibres may be seen passing to the subjacent
musculature. These fibres are not aggregated into roots.

[v.03 p.0238] [Illustration: FIG. 2.--Structure of branchial region.

_bc_, coelom.
_tb_, tongue-bars.
_ds_, mesentery.
_pr_, ridge.
_vv_, vessel.
_gp_, gill-pore.
_dn_, dorsal nerve.
_dv_, vessel.
_oe_, oesophagus.
_vs_, mesentery.
_vn_, ventral nerve. ]

_Gill-slits_.--The possession of gill-slits is as interesting a feature in
the organization of _Balanoglossus_ as is the presence of tracheae in
_Peripatus_. These gill-slits occupy a variable extent of the anterior
portion of the trunk, commencing immediately behind the collar-trunk
septum. The branchial bars which constitute the borders of the clefts are
of two kinds:--(1) Septal bars between two contiguous clefts, corresponding
to the primary bars in _Amphioxus_; (2) Tongue-bars. The chief resemblances
between _Balanoglossus_ and _Amphioxus_ in respect of the gill-slits may be
stated briefly as follows:--([alpha]) the presence of two kinds of
branchial bars in all species and also of small crossbars (synapticula) in
many species; ([beta]) numerous gill-slits, from forty to more than a
hundred pairs; ([gamma]) the addition of new gill-slits by fresh
perforation at the posterior end of the pharynx throughout life. The chief
differences are, that (a) the tongue-bar is the essential organ of the
gill-slit in _Balanoglossus_, and exceeds the septal bars in bulk, while in
_Amphioxus_ the reverse is the case; (b) the tongue-bar contains a large
coelomic space in _Balanoglossus_, but is solid in _Amphioxus_; (c) the
skeletal rods in the tongue-bars of _Balanoglossus_ are double; (d) the
tongue-bar in _Balanoglossus_ does not fuse with the ventral border of the
cleft, but ends freely below, thus producing a continuous U-shaped cleft.
The meaning of this singular