Lining the sides of the gill arches are rows of
regularly spaced filaments projecting posteriolaterally.
Each row or stack of filaments constitutes a
hemibranch, while a set of hemibranchs, one on
each side of the arch, constitutes a holobranch.
The gills of teleost fishes are composed of four such
holobranchs spaced between five branchial slits
(chambers) (Fig. 1d) while the elasmobranchs
have an extra hemibranch on the anterior side of
the first branchial slit (Fig. 1c)