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Phacops sp. eye

Trilobites ("three lobes") are extinct arthropods. They were the first animals known to have eyes, although some were secondarily blind. They were very numerous during the early Palaeozoic era.

Their distribution was worldwide, but only in salt water environments. They disappeared in the Permian–Triassic extinction event.

Trilobites had many lifestyles. Some moved over the sea floor as predators, scavengers, or filter feeders. Some swam, feeding on plankton. Most ecological niches of modern marine arthropods are seen in trilobites.

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.