To achieve this fish use gills, water is drawn through the mouth and expelled over the gills and out via the operculum. As the water passes over the gill membranes oxygen is absorbed into fine capillaries. S ome species have adapted and can breathe atmospheric air when required. The Anabantidae group, often called Labarinth Fish have developed a labyrinthine organ in the head for this purpose and several species of catfish utilise the gut to make use of swallowed atmospheric air. W hile the shape of the fish plays a great part in its efficiency as a swimmer, the force that enables it to move comes from the co-ordination of body flexing and fin movements.
Fins consist of bony supporting rays with soft webbing tissue between.
The dorsal and anal fins are set in the vertical plane and act very like the keel of a boat, keeping the fish upright. The two sets of paired fins, the pectoral and pelvic or ventral fins are for manoeuvring and breaking. The pelvic fins act as hydroplanes, supporting the fish at rest. Seen from above, the fish moves forward by a means of a series of S-shaped flexings of the body which culminate in a thrust from the caudal fin. A s well as their evolved weaponry, fish have what may be called a primary defence system.