Article: The visual system of trilobites [Twenty-first Annual Address, delivered 8 March 1978]

The compound eyes of trilobites are the oldest of all known visual systems and their evolution can be traced over 350 million years. Only the lentiferous surface is preserved, however, since the lenses alone are calcitic.Holochroal eyes have many lenses closely packed together. From a study of their evolution, the morphology and optics of their lenses, various systems of lens packing, and the relationships between lens-thickness and that of the cuticle, it is possible to disentangle those features of the lentiferous surface which result purely from geometrical growth constraints from those which may have been of physiological significance. Holochroal eyes probably functioned in a manner analogous to that of many modern insect and crustacean eyes.Schizochroal eyes, unique to the animal kingdom, have large separated lenses. They probably were derived by paedomorphosis from a holochroal precursor. The complex internal structures of these lenses has been investigated using optical and scanning electron microscopy, as well as cathodoluminescence, which has enabled primary to be distinguished from secondary structures.Each lens, like those of holochroal eyes, when sectioned parallel with its principal plane shows calcite fibres arranged in lamellae radiating from the central axis. Sections cut along the axis, however, show first how the lower part of the lens contains an intralensar bowl of different texture to the rest of the lens, and secondly that the radial lamellae are constructed of calcite fibres (trabeculae) diverging fanwise from the axis in the upper part of the lens, to abut the upper convex surface near normally.The complex internal structure of the schizochroal lens seems both to minimize birefringence, and to correct for spherical aberration. Such high-quality lenses must have been linked to a photoreceptive system capable of making use of their sharp images; in this and other contexts various theories of optical function in schizochroal eyes are discussed and analysed.