Jawless fish are marine vertebrates that evolved over 50 million years ago. Most are extinct except for lampreys and hagfish. They are primitive looking, and lack scales or paired fins. They have gill pouches for ventilation and slits on either side of their body and back of the head. They have cartilaginous skeleton rather than bones and their body is eel-like. They lack a jaw bone; instead, they have a round open mouth called an oral disk. The lamprey survives in the ocean by attaching themselves to other fish by their mouth and sucking the blood of other fish. The hagfish survives by secreting a mucous slime that makes other predators back off. Hagfish eat dead animals and burrow into their carcasses with their mouths.
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