It is characterised by a mobile connection of the upper jaw to the skull. The premaxillary and often the maxillary bones are free and can move forward, forming a mouth tube through which the fish sucks in food organisms. Such a mouth is typical of fish that feed on plankton (herring), benthos (bream, carp) or detritus (mullet). Many fish (sturgeon, carp) have a retractable mouth that allows them to dig easily in the mud in search of food.

With an immobile mouth structure, the upper jaw is attached to the skull motionless or almost motionless: predators; benthophages that feed on mollusc shells, hard shells of crustaceans and echinoderms.