Cartilaginous fish have an internal cartilaginous skeleton. Their characteristic features are also several pairs of gill slits that open outward, and the absence of a swim bladder. Almost all modern cartilaginous fish live in the seas. Among them, sharks and rays are the most common. Most modern fish have an ossified internal skeleton and belong to the class Bony fish. A pair of external gill slits are covered with gill covers. Many bony fish have a swim bladder.
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