Three main types of eye have been defined in trilobites; holochroal, schizochroal and the rare abathochroal. At least in holochroal and schizochroal eyes, the lenses consist of oriented calcitic microcrystallites, the so-called trabecula, which run uninterruptedly throughout the lens from top to bottom. It is argued here that these are primary structures and not diagenetic. Holochroal eyes are commonly accepted to correspond to apposition eyes, which are the most common type of compound eyes in arthropods living today. Schizochroal eyes, present only in phacopine trilobites, are characterised by a doublet lens structure, with an aplanatic interface, correcting spherical aberration of the thick lenses to form a sharp focus for incident rays travelling parallel with the optical axis. This classic model seems to be functional for many phacopines. In these the trabecula are present, though all juxtaposed, forming a solid block. In the lenses of some phacopid species, however, the microcrystallites are separated from each other by gaps. If, during life, these gaps were filled with organic material, as the lens grew from the larval stages, or post-ecdysially, each trabeculum would be isolated from its neighbours by an organic sheath. A simple model is proposed here for the generation of the organic sheath surrounding each trabeculum. If the individual trabecula were isolated from each other, then the possibility of a new kind of visual system exists in these ‘derived’ phacopines. The differences between the refractive indexes inside and outside the trabeculum would ensure that each trabeculum acted as a light-guide, so that the whole ‘lens’ becomes a light guide bundle. This would result in a pixelled visual system, unique in the animal realm, but an archetype for modern technology of data transfer.
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