Gill slit

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Gill slits are gills with individual openings rather than an outer cover. Cartilaginous fish such as sharks, rays, sawfish, and guitarfish all have gill slits. Most have five pairs, but a few species have 6 or 7 pairs. Bony fish have an outer bony gill covering called an operculum.

The term "gill slits" also refers to the folds of skin in all vertebrate embryos. The skin folds in mammals, birds, and reptiles are gill slits, but the gill slits in embryonic fish develop into gills, while 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.