Internal and External Gills

In living fish, gill arches support the gills. These structures are derived from the embryonic pharyngeal arches. Water enters the mouth, moves through internal gill slit in the pharynx and into the gill pouches which are lined by gills bearing gill filaments. In sharks, each gill pouch opens separately to the exterior and can be covered by a septum. In bony fish, all gills lie within a single pouch which is covered by the operculum. In animals with external gills (mainly larvae of fish and amphibians) the gills project outward into the water. Note also that only Chondrichthyes fish (illustrated by the shark) have spiracles.