The first aquatic vertebrates, the Agnatha, which appeared 450 million years ago in the Ordovician period, are represented today solely by the lampreys and hagfishes. Early agnathans were quite different in appearance from the lampreys. Many had a bony armour covering the head and trunk and were dorsoventrally flattened, suggesting a slow-moving bottom-living habit, while others were evidently more active, with bony scales and a slender body. All these ancient fishes had a median pineal eye on top of the head. This is retained in the lamprey, but no trace remains of ancestral bony armour in any present-day agnathan.
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