6 Facts About Hammerhead Sharks
Pisces In some classifications, a superclass of the Gnathostomata (jawed chordates) making up the fishes (compare Tetrapoda). The whole body of a fish is covered with a difficult, normally scaly, skin (see scales), which extends over the eye and also includes pigments and also in some cases slime glands. The circulatory system is a single circuit with blood passing with two sets of blood vessels, one at the gills and also the other in the body tissues. Three semicircular canals are present in the inner ear. The sense of odor is especially well established and also pressure waves are found by the lateral-line system (see lateral-line canal). Fossils of fish date back to the Ordovician period, 505-- 438 million years ago. There are two courses of modern fish: Chondrichthyes (cartilaginous fishes) and also Osteichthyes (bony fishes).
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