The earliest fish were very small. They were heavily armoured to protect themselves from attacks by the invertebrate predators that were common in the seas 450 million years ago. Most of these fish seem to have lived by feeding on small animals and algae in the mud on the sea-bed. Some of the earliest forms had a third eye and all of them differed from modern fish because they had no jaws. Their mouth was a simple hole or slit, which limited the types of food they could eat. The descendants of these primitive fish are modern lampreys and hagfish. These eel-like creatures resemble their ancestors in that they have no jaws. However, they are either parasites living off other fish or scavengers feeding on dead fish.
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