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. Boney fish have an outer boney gill covering called an operculum.

The term "gill slits" also refers to the folds of skin in many vertebrate embryos. The skin folds in mammals, birds, and reptiles are not gills, 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. This is partly why it is thought that the gills of fish evolved into the ears of vertebrates.

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