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Fish

Lampreys and hagfish

Pacific hagfishPacific hagfishLampreys and hagfish are ancient species that resemble fish in their early stages of evolution, more than 300 million years ago. Neither are considered to be true vertebrates: instead of a true backbone, lampreys have a series of cartilaginous (gristly) structures running down their backs, while hagfish have no vertebrae at all—just a “skeleton” made of cartilage. Both have eel-like elongated bodies, and, just like the earliest kinds of fish that evolved hundreds of millions of years ago, they have no jaws.

Lamprey mouthLamprey mouth

Lampreys

There are 38 known species of lamprey. They have a toothed, funnel-like, sucking mouth which they use to bore into the flesh of other fish to suck their blood. Like eels, adult lampreys have scale-less, elongated bodies, ranging between 13 and 100 centimetres (5–40 inches) in length. They have large eyes, a single nostril on top of their head and seven gill holes on each side of their body.

Lampreys live mostly in temperate regions. Adults spawn in rivers and then die. The larvae spend several years in rivers, burrowed in sediments and feeding on microscopic organisms. After undergoing a metamorphosis, some species migrate to the sea or lakes, where they feed on other fish or marine mammals.
European river lampreysEuropean river lampreys 

Hagfish are the only known living animals that have a skull but no vertebral column.

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