Most fish rely on gills to take in oxygen. Gills can be covered with flaps, hidden under slits, or carried externally. Take a look at the external appearance of the gills and then take a closer look at the frilly gills themselves.
Covered gills - Typical fish cover their gills with a flap called an operculum, leaving one opening at the flap on each side of the fish's head. Bluegill has a distinctive "black ear flap" on its operculum. Are those actually "ear flaps"?
Photographer - TBA. Get permission.
Uncovered gill slits - This Gray Nurse Shark (Carcharias taurus) displays the uncovered gill slits found on sharks, rays and skates. Although sharks can have more gill slits, most sharks have five.
Photographer - Richard Ling.
Permission by Creative Commons 2.0
Uncovered gill slits - The gills on a Frilled Shark (Chlamydoselachus anguineus) are partly exposed.
Most land animals have lungs, and fish have gills.
Things to note:
Oxygenated air flows in and out of lungs while oxygenated water flows through ...
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This Wikipedia and Wikimedia Commons image is from the user Chris 73 and is freely available at http://commons.wikimedia.org/wiki/Image:Tuna_Gills_in_Situ_01.jpg under the creative commons cc-by-sa 2.5 license.
Gills of unspecified fish
Underneath the operculum or flap that covers the gills of fish, you can see both the gill rakers (white comb-like structure) and the attached gill filaments (red fringe). Both of these are attached to the bony structure of the fish by the bony gill arch that is hidden in this image. Water enters throught the fish's mouth, particles are filtered by the gill rakers, passes over the gill filaments where oxygen flows in to the circulatory system like in our lungs, and passe outside through the gill flap.