In some classifications, a superclass of the Gnathostomata (jawed chordates) comprising the fishes (compare tetrapoda). The whole body of a fish is covered with a tough, usually scaly, skin (see scales), which extends over the eye and contains pigments and sometimes slime glands. The circulatory system is a single circuit with blood passing through two sets of capillaries, one at the gills and the other in the body tissues. Three semicircular canals are present in the inner ear. The sense of smell is particularly well developed and pressure waves are detected by the lateral-line system. Fossils of fish date back to the Ordovician period, 505–438 million years ago. There are two classes of modern fish: Chondrichthyes (cartilaginous fishes) and Osteichthyes (ray-finned, or bony fishes).

https://australianmuseum.net.au/Fishes The Australian Museum Fish Site, an extensive site catering for fish enthusiasts at various levels