Gill slit

Such gills are characteristic of cartilaginous fish such as sharks and rays, as well as deep-branching vertebrates such as lampreys.

The anterior edge of a gill slit is motile, moving outward to allow water to exit, but closing to prevent reverse flow.

[1] While the shark is moving, water passes through the mouth and over the gills in a process known as "ram ventilation".

A small number of species have lost the ability to pump water through their gills and must swim without rest.

The term "gill slits" has also been used to refer to the folds of skin in the pharyngeal region in embryos.