Gills - branchiae - facts

   The gills or branchiae are the respiratory organs of aquatic animals. In the invertebrates especially there is great variety in the structure of gills, which may be situated almost anywhere on the body and may vary from simple skin pouches of echinoderms to the complex fimbriated (fringed) and enclosed type of gills of crayfish and lobster. All invertebrate gills are continuations of the body wall and are generally external, although they may be protected in various ways. In free-swimming worms, such as Nereis, the common sandworm, the gills are a series of tufts along each side of the body.
fish gills
   There are many insects with aquatic larvae. These have tracheal gills that are usually thin, feathery, leaflike or threadlike expansiona of the chitinous exo-skeleton (tough, protective body covering), and are usually located on the abdomen. A few aquatic in­sects have covered gills and some have developed respiratory tubes with which they penĂ©trate the surface film of water to get air.
   The gills of lamellibranch mollusks, such as the oyster, are chiefly food-gathering devices and have virtually lost their respiratory function, which is performed by the mantle.
   The vertebrate gill is entirely different in its origin and structure. Typically vertebrate gills are developed as,a set of thin, fimbriated plates, richly supplied with blood vessels and situated in a gill cleft made by an outpouching of the pharyngeal wall meeting a lesser invagination in the outer body wall. Such gill pouches are paired and located in the pharyngeal region. Each set of gill filaments is supported by a cartilaginous skeleton, the gill arch. There is good evidence in the young lamprey and in the prevertebrates, i.e., Amphioxus, that pharyngeal gills originated as food-gathering devices and assumed their respiratory func­tion as a secondary development. The young of elasmobranchs, lungfish, and amphibians have an external type of gill, developed from the external body wall.
   All vertebrales, including man, possess gill pouches in their embryological development and all land vertebrales retain the first of these pouches, the eustachian tube, as a conneclion between the inner ear and the pharynx. Others may occasionally break through as anomalies (deviations from the representative characterislics of the group).
   Jaws, one pair of the ear bones, and the tracheal cartilages of the vertebrates appear to have developed from gill arches. Many aquatic animals that have never developed gills, or have lost them, respire directly through the body wall.