What are trilobites?
Trilobites are an extinct group of marine arthropods, which lived over 350 million years ago during the Palaeozoic era from the Lower Cambrian until the Late Permian.
All trilobites share a common body plan: a head (cephalon), a body (thorax) and tail (pygidium). The body can also be divided in three length-ways, into two outer pleural lobes and a central axial lobe, and it is on this division that the name trilobite is based.
Trilobite eyes are the most ancient visual system known. The eyes are compound like the eyes of crustaceans and insects today. They are made up of many tiny lenses. Some trilobites were blind; for example the Agnostina were primarily eyeless.
legs are rarely preserved and are known in only a few specimens. The legs show remarkable consistency. There is always a first pair of uniramous antennae, followed by paired biramous (two-branched) appendages under the cephalon. There is one pair for each segment down the body and pygidium. The basal joint of the appendages gives rise to two branches, the walking leg and a gillbranch (seen in the lower part of this diagram).

Websites produced by students on the MSc Palaeobiology programme in the Department of Earth Sciences at the University of Bristol for academic year 2003-4