External gills are temporary, specific respiratory organs that function during the larval stage of fish development and disappear after the formation of the definitive organs.
External gills usually consist of a single stem (rami) projecting from the gill arch behind the heads, above the corresponding gill slit. The stalk usually contains muscular tissue and can move as a free appendage to stir up stagnant water. The stalk is lined with many thin-walled filaments (fimbriae) containing most of the blood vessels used for gas exchange. Animals usually have one external gill beginning at each gill arch (except the hyoid arch), resulting in three pairs of external gills in salamanders and four in the gill larvae of double-breathing fish.
By nature, they are probably modified gill lobes of internal gills that have migrated outward. In celachians, external gills are present only in the embryonic state in the form of branching appendages of the gill lobes emerging from the gill slits. External gills between ganoids were observed in young Crossopterygii (Polypterus and Calamoichtys), whose gill cover is elongated into a back-thinning appendage bearing an upper and lower row of filamentous lobes with lateral processes; the arrangement of individual gill parts resembles a feather. Bonygids Gymnarchus and Heterotis have in youth filiform gills, reminding gills of germ of celacia. External gills in dicotyledons are observed in young Lepidosiren and Protopterus in a form of four pairs of lamellate appendages with lateral branches behind operculum. In Protopterus, only the anterior gill atrophies, while the three posterior ones remain for life as appendages and lie one above the other, shifting to the shoulder girdle area. In Lepidosiren H. gills are atrophied, and in Ceratodus they do not appear at all. In amphibians, H. gills may have the form of simple lamellar appendages, as in Dactyletra larvae, or comb-like appendages, i.e., with one row of lateral branches, as in most tadpoles of tailless amphibians, or pinnate appendages, i.e., two rows of branches. i.e., with two rows of branches, as in Derotremata larvae, or, in the form of plates covered by nonbranched (Axolotus, Menobranchus) or branched (Proteus, Siren) appendages arranged in several rows. The larvae of legless amphibians also have pinnate gills (Ichtyophis) or the latter take the form of two huge, vessel-rich lobes (Coecilia). Sometimes rudimentary tubercles of external gills are present in Gymnophiona on the mandibular and hyoid arch. Usually amphibians have external gills above the three anterior gill arches in three pairs. In all, except Perennibranchiata, they are pectinate, atrophied.
Tags: external gills