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Fish are aquatic vertebrates that have skin covered with scales, two sets of paired fins, some unpaired fins, and a set of gills. Fish do not represent a single clade but are instead paraphyletic. They include hagfishes, lampreys, lobe-finned fishes, cartilaginous fishes and ray-finned fishes.
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Bony fishes (Osteichthyes) are a group of aquatic vertebrates characterized by having a bony skeleton (in contrast to the cartilaginous fishes, a group whose skeleton consists of cartilage, not bone). Bony fishes include two sub groups, the ray-finned fishes and the lobe-finned fishes.
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Cartilaginous fish are so named because instead of bony skeletons, their body frame consists of cartilage. Tough and flexible, cartilage provides enough structural support to enable these fish to grow to incredible sizes. Cartilaginous fish include sharks, rays, skates and chimaeras.