Gills are tissues that are like short threads, protein structures called filaments. These filaments have many functions including the transfer of ions and water, as well as annales Des Sciences Naturelles. Zoologie, Volume 9 PDF exchange of oxygen, carbon dioxide, acids and ammonia. Juvenile bichirs have external gills, a very primitive feature that they share with larval amphibians.
Previously, the evolution of gills was thought to have occurred through two diverging lines: gills formed from the endoderm, as seen in jawless fish species, or those form by the ectoderm, as seen in jawed fish. Tuna gills inside of the head. The fish head is oriented snout-downwards, with the view looking towards the mouth. Air breathing fish can be divided into obligate air breathers and facultative air breathers. Obligate air breathers, such as the African lungfish, are obligated to breathe air periodically or they suffocate.
All basal vertebrates breathe with gills. The gills are carried right behind the head, bordering the posterior margins of a series of openings from the esophagus to the exterior. Each gill is supported by a cartilagenous or bony gill arch. The gills are composed of comb-like filaments, the gill lamellae, which help increase their surface area for oxygen exchange. When a fish breathes, it draws in a mouthful of water at regular intervals. Then it draws the sides of its throat together, forcing the water through the gill openings, so that it passes over the gills to the outside. Gills usually consist of thin filaments of tissue, branches, or slender tufted processes that have a highly folded surface to increase surface area.