Respiratory organs - gill apparatus, providing oxygen to the body. The shape of the gills depends on the species and mobility of fish. These are bags, on the inner side of which are located gill lobes (fish-like), septa with folds of mucous membrane (cartilaginous), gill arches with lobes (bony). Unfavorable oxygen conditions promoted the development of additional respiratory organs - swim bladder (double-breathing fish), supra-gill labyrinths (snakehead, perch-crawler, jumping fish), skin (catfish, eel, crucian carp, tench, carp), intestines (youyun, puffer, some gobies).