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| | Information on Fish Anatomy |
 | | The living species of fish are usually divided into three classes: the Agnatha, the jawless fishes, comprising the hagfishes and lampreys; the Chondrichthyes, the cartilaginous-skeleton fishes, such as sharks and rays; and the Osteichthyes, the bony-skeleton fishes, comprising all other living fishes. |
 | | In primitive bony fishes, such as the sturgeon, the vertebrae spaced along the notochord are still largely cartilaginous, but in most advanced bony fishes the vertebrae are bony and are united to form the backbone, and the notochord is no longer present. |
 | | The dorsal and anal fins may be supported by cartilaginous rods, as in the lampreys, by cartilaginous rods and horny rays, as in sharks, by horny rays, as in the spiny-finned fishes, or by bony rays (derived from scales) in the soft-rayed fishes. |
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