The analogous structure to the gill filament in
the hagfish is the primary gill fold. The primary
gill folds are arranged radially around the central
axis of the pouch like the lines of longitude on a
globe (Fig. 1e). Some filaments extend to the
centre of the pouch while others have a low profile.
The filament can be subdivided into afferent,
respiratory, and efferent regions (Fig. 4). In the
respiratory region, lamellae are present as subdivisions
and branchings of the filament through
third to seventh order folding. The lamellae in
hagfish are orientated in the same plane as the
filament, which is unlike the arrangement in
lamprey and other fishes in which the lamellae
are perpendicular to the long axis of the filament.
Water and blood flows are counter-current.