The Ventral Part of the Hyoid Gill-Slit and a Mandibular 'Operculum' in some Bony Fishes

Abstract

(1) The hyoid gill-slit. It is commonly accepted that in all living fishes there is no sign of a ventral part of the hyoid (facial) gill slit, that the latter has vanished entirely, and that all that is left of it is a small relic of its dorsal extremity which survives as a spiracle in some primitive forms. Nevertheless, all fishes of the family Malacosteidse (Stomiatoidea) have a large, wide open slit between the mandible and the ventral part of the hyoid arch, that is, corresponding exactly to the ventral part of the hyoid (facial) gill-slit. The fact itself was known long ago, but was described in somewhat obscure terms, and its significance was overlooked.