<P> 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, as well as primitive fish such as lampreys . In contrast, bony fishes have a single outer bony gill covering called an operculum . </P> <P> Most sharks and rays have five pairs of gill slits, but a few species have 6 or 7 pairs . Shark gill slits 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 . A 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 . </P> <P> 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 . </P>

Why is the number of pharyngeal gill slits in sharks
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