Editing Chondrichthyes (section)

===Respiratory system===
All chondrichthyans breathe through five to seven pairs of [[gill]]s, depending on the species. In general, pelagic species must keep swimming to keep oxygenated water moving through their gills, whilst demersal species can actively pump water in through their [[Spiracle (vertebrates)|spiracles]] and out through their gills. However, this is only a general rule and many species differ.

A spiracle is a small hole found behind each eye. These can be tiny and circular, such as found on the nurse shark (''Ginglymostoma cirratum''), to extended and slit-like, such as found on the wobbegongs (Orectolobidae). Many larger, pelagic species, such as the mackerel sharks (Lamnidae) and the thresher sharks (Alopiidae), no longer possess them.