The cuticle of Agnostina (basal Euarthropoda) was three-layered

Abstract

The cuticle of four Agnostina families is notably complex, challenging the previously proposed one-layer model. It consists of three distinct layers, with the outer layer being significantly thicker than the middle and inner layers; two upper layers are laminated. The inner layer is very thin and rests on a base of fibrils arranged in a rhomboidal mesh. Comparative analysis suggests that during evolution, the middle and inner layers have decreased in thickness, leaving the outer layer relatively thick. These distinctive structural features are possibly associated with miniaturization of agnostines. Their cuticle resembles that of chelicerates more than that of crustaceans.