Cuticle and cuticular sensilla in Agnostina
The microstructure of the cuticle of several Agnostina species is described. For the first time, it has been shown that their cuticle consists of three layers: thick outer and transitional layers and a thin inner layer. The layers are made up of stacks of laminae: the laminae in the outer layer are thicker than in the transitional and inner ones. The evolution of the agnostoid cuticle proceeded, apparently, in the direction of decreasing the thickness of the inner layer which in contrast to all other arthropods, is thinner than the outer layer. Apparently, the inner layer was mineralized to a lesser extent than the outer one. In the structure of the cuticle, Agnostina are close to horseshoe crabs and trilobites. On the outer and inner layers, a set of sensilla was revealed; they can be attributed to trichoid, campaniform, and digitiform sensilla (with mechanoreceptor function) and celoconic sensilla (with chemoreceptor function). Also, unique sensorial ensembles were found on cephalons and pygidia; the function of the former is probably mechanosensorial, and the purpose of the latter cannot be assumed on this material.