Gill slits are individual openings to gills, i.e., multiple gill arches, which lack a single outer cover. Such gills are characteristic of cartilaginous fish such as sharks, and rays. Most of these have five pairs, but a few species have 6 or 7 pairs. The anterior edge of a gill slit is motile, moving outward to allow water to exit, but closing to prevent reverse flow. In contrast, Bony fishes have a single outer bony gill covering called an operculum.
The term "gill slits" has also been used to refer to the folds of skin in the pharyngeal region in embryos. The true gill slits in embryonic fish develop into gills. However, the slits in tetrapods do not, so a better name for the vertebral structures is pharyngeal slits. It is now accepted that it is the vertebrate pharyngeal pouches and not the neck slits that are homologous to the pharyngeal slits of invertebrate chordates (Pharyngeal slit). The origins of gill slits originates from primal tunicet species, there the pharingeal slits were used as filter feeding organs
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