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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, rays, sawfish, and guitarfish. 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 a single outer bony gill covering called an operculum.

The term "gill slits" is also sometimes used to refer to the folds of skin in the pharyngeal region in all vertebrate embryos, including those of humans. However, the term gill suggests a particular anatomical structure or function, and the "gill slits" in amniotes have neither. Therefore, a better modifier is pharyngeal, as in pharyngeal arches or pharyngeal slits.

The skin folds in mammals, birds, and reptiles are sometimes called "gill slits", but the true gill slits in embryonic fish develop into gills, whereas the "gill slits" in other vertebrates develop into the throat area and the bones in the ear. "Gill slits" exist in all vertebrates at some time in their embryo stage. When the vertebrate reaches a certain point in its life the skin folds fuse together and the bones form the trachea and ear bones. Except in fish where they form the gills themselves.

In the 19th century, "gill slits" of vertebrate embryos were erroneously thought to be actual gills, and thus evidence for the recapitulation theory.

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