Trilobite segmentation

Abstract

Various lines of evidence indicate that the trilobite cetphalon is composed of six somites in addition to the presegmental acron. Five of the somites are provided with typical appendages, four pairs of legs and one pair of antennae. The labrum (and its sclerite, the hypostome) appears to be formed, wholly or partly, by a sixth pair of modified preantennal appendages. The cephalic-thoracic boundary in trilobites as well as in most Palaeozoic xiphosurids is thought to be roughly intersegmental, and the thoracic sclerites probably largely correspond to somites. The concept of merocyclism is rejected.