Trilobites ("three lobes") are extinctarthropods. They were the first animals known to have eyes, although some were secondarily blind. They were very numerous during the early Palaeozoicera.
Their bodies were divided into three parts: the head (cephalon), the thorax (chest) made of up to 30 segments, and the tail (pygidium). Underneath, and rarely preserved, are three pairs of legs for the head. There are paired legs for each pleural groove.