What is "gill slit"

Gill \Gill\ (g[i^]l), n. [Dan. gi[ae]lle, gelle; akin to Sw. g["a]l, Icel. gj["o]lnar gills; cf. AS. geagl, geahl, jaw.]

1. (Anat.) An organ for aquatic respiration; a branchia.

   Fishes perform respiration under water by the gills.
   --Ray.

   Note: Gills are usually lamellar or filamentous appendages, through which the blood circulates, and in which it is exposed to the action of the air contained in the water. In vertebrates they are appendages of the visceral arches on either side of the neck. In invertebrates they occupy various situations.

2. pl. (Bot.) The radiating, gill-shaped plates forming the under surface of a mushroom.

3. (Zo["o]l.) The fleshy flap that hangs below the beak of a fowl; a wattle.

4. The flesh under or about the chin.
   --Swift.

5. (Spinning) One of the combs of closely ranged steel pins which divide the ribbons of flax fiber or wool into fewer parallel filaments. [Prob. so called from F. aiguilles, needles.
   --Ure.]

   Gill arches, Gill bars. (Anat.) Same as Branchial arches.

   Gill clefts. (Anat.) Same as Branchial clefts. See under Branchial.

   Gill cover, Gill lid. See Operculum.

   Gill frame, or Gill head (Flax Manuf.), a spreader; a machine for subjecting flax to the action of gills.
   --Knight.

   Gill net, a flat net so suspended in the water that its meshes allow the heads of fish to pass, but catch in the gills when they seek to extricate themselves.

   Gill opening, or Gill slit (Anat.), an opening behind and below the head of most fishes, and some amphibians, by which the water from the gills is discharged. In most fishes there is a single opening on each side, but in the sharks and rays there are five, or more, on each side.

   Gill rakes, or Gill rakers (Anat.), horny filaments, or progresses, on the inside of the branchial arches of fishes, which help to prevent solid substances from being carried into gill cavities.

Slit \Slit\, n. [AS. slite.] A long cut; a narrow opening; as, a slit in the ear.

Gill slit. (Anat.) See Gill opening, under Gill.

n. (context anatomy fish English) An individual opening to gills, which lacks an operculum; characteristic of Cartilaginous fish such as sharks.

n. one of a series of slit openings in the pharynxes of fishes and aquatic amphibians through which water passes [syn: branchial cleft, gill cleft]

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, and rays. Most of these have five pairs, but a few species have 6 or 7 pairs. Shark gill slits are not covered, but lie in a row behind the head. The anterior edge of a gill slit is motile, moving outward to allow water to exit, but closing to prevent reverse flow. This modified slit, called a spiracle, lies just behind the eye, which assists the shark with taking in water during respiration and plays a major role in bottom–dwelling sharks. Spiracles are reduced or missing in active pelagic sharks. While the shark is moving, water passes through the mouth and over the gills in a process known as "ram ventilation". While at rest, most sharks pump water over their gills to ensure a constant supply of oxygenated water. A small number of species have lost the ability to pump water through their gills and must swim without rest. These species are obligate ram ventilators and would presumably asphyxiate if unable to move. Obligate ram ventilation is also true of some pelagic bony fish species. In contrast, bony fishes have a single outer bony gill covering called an operculum.

The true gill slits in embryonic fish develop into fish gills. However, the slits in tetrapods do not, so a more general name for the vertebral structures is pharyngeal slits. Gill slits likely originated from pharyngeal slits in tunicates that were used for filter-feeding. The term "gill slits" has also been used to refer to the folds of skin in the pharyngeal region in embryos. It is now accepted that it is the vertebrate pharyngeal pouches and not the neck slits that are homologous to the pharyngeal slits of invertebrate chordates.