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 shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....

s, rays
Batoidea is a superorder of cartilaginous fish commonly known as rays and skates, containing more than 500 described species in thirteen families...

, sawfish
Sawfish, also known as the Carpenter Shark, are a family of rays, characterized by a long, toothy nose extension snout. Several species can grow to approximately . The family as a whole is largely unknown and little studied...

, and guitarfish
The guitarfish are a family, Rhinobatidae, of rays. The guitarfish are known for an elongated body with a flattened head and trunk and small ray like wings. The combined range of the various species is tropical, subtropical and temperate waters worldwide. They often travel in large...

. 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
Operculum (fish)
The operculum of a bony fish is the hard bony flap covering and protecting the gills. In most fish, the rear edge of the operculum roughly marks the division between the head and the body....


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
Recapitulation theory
The theory of recapitulation, also called the biogenetic law or embryological parallelism—and often expressed as "ontogeny recapitulates phylogeny"—is a disproven hypothesis that in developing from embryo to adult, animals go through stages resembling or representing successive stages...


These pharyngeal archs exist in all vertebrates at some time in their embryo stage (in mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...

s, bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s, and reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s), 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.
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