The teleosts are
characterized by the universal presence of a bony
operculum. The operculum develops from a fold in
the hyoid arch and is a hard structure composed of
dermal bone, with a crescent-shaped caudal opening
(Fig. 1d). The ventral edges are connected by
an expandable membrane beneath the pharynx,
and the space enclosed is the opercular cavity or
space. The presence of the operculum likely led to
the loss of the first hemibranch in the teleosts
since water delivery to this area is poor (Romer
and Parsons, ’86). The chimeras, which are also
members of the chondrichthyes, differ markedly
from the elasmobranchs in the gross morphology
of their gills and associated structures. The
numerous gill slits are replaced by a single
opening covered by a large fleshy operculum that
extends caudally. The interbranchial septa are
also shorter, the last branchial chamber is closed,
and the spiracles are lost. In these respects, they
more closely resemble the teleosts.