Definition: Trilobites

Trilobites form a class of primitive marine arthropods. This group is characterised by the possession of antennae and biramous appendages and locomotive appendages bearing gills. Trilobites are all fossils: they are known to have existed throughout the Palaeozoic era, from the Lower Cambrian to the Upper Permian (Murghabian). They owe their name to the division of their bodies into three parts, from front to back (cephalon, thorax, pygidium), and laterally (glabella and rachis separating the two pleurae).