Gill slit

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 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 vertebrate do not, so a better modifier for the vertebral structures is pharyngeal, as in pharyngeal slits and pharyngeal arches. In the 19th century, "pharyngeal slits" of vertebrate embryos were erroneously thought to be actual gills or precursor slits to gills, so thought then to be support for the recapitulation theory.

These pharyngeal archs exist in all vertebrates at some time in their embryo stage (in mammals, birds, and reptiles), and develop into a number of permanent structures in and away from the throat area and the bones in the ear. When the vertebrate reaches a certain point in its life the arches fuse together and form other structures such as the trachea and ear bones, and contribute to major blood vessels.

Despite evidence to the contrary some books on evolution still use the supposed gill slits as evidence for evolution. In South Africa in their grade 12 text books this error still appears as evidence that mammal embryo's go through a fish like stage. (Life Sciences Grade 12 Textbook and Workbook by J Grogan & R Suter published April 2010) This is scientifically wrong.