Cuticular structure of the agnostine trilobiteHomagnostus obesus

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Abstract
Study of the exoskeletal surface microstructure of specimens of Homagnostus obesus (Belt, 1867) from the Upper Cambrian of Sweden has given information about the structure of agnostine cuticle. It is likely that the very thin cuticle of agnostines (5-15 .mu.m), unlike that of polymerid trilobites, was constructed only of a prismatic layer. The exoskeletons were strengthened by reticulation on the external surface, the ridges forming up to 15% of the total cuticle thickness. Pits on the visceral surface of the exoskeleton of H. obesus may have contained photoreceptors as their morphology is similar to that of the Nileus glabellar ''tubercle''. This would have allowed the animal to monitor changes in light intensity. Possible sensory receptors in other agnostine trilobites are reviewed. Most sense organs were positioned on the unmineralized ventral surface of the organism.