Although, with rare exceptions, we have no knowledge of the interna] structure of the eyes of trilobites, the fossil record yields astounding evidence of the dioptric apparatus, the outermost region of the ommatidia. This is due once again to the fact that only this portion was sufficiently mineralized to remain preserved in the fossilization process. In favorable circumstances, the eye lenses survived intact their long burial: they were already made of calcite crystals in the living animals! Not all trilobites possessed eyes—some, in fact, were blind; but others had enormous eyes which would take up most of the cephalic surface. Often the eyes were shaped in turretlike fashion, and their combined visual field could cover the animal's entire surroundings. Trilobite eyes, as we shall see, are revealing indicators of the habits of their carriers. Lacking the complete structure of the ommatidia, we cannot, of course, draw an exact analogy between the compound eyes of trilobites and those of modern arthropods. However, the external appearance and the lensar arrangement are often suggestive of a very close correspondence. If provided with the present knowledge, Exner would probably recognize apposition and superposition eyes in trilo-bites. Lindstrom (1901) instead recognized two different kinds of trilobites' eyes: a truly compound eye and an aggregate eye. The compound, or holochroal, eye is characterized by a close packing of the lens units, the entire visual surface being covered by a continuous pellucid membrane, the cornea. The lenslets vary in shape from thin biconvex to elongated hexagonal prisms. A different structure is presented by the aggregate, or schi%ochroal, eye. Here the lenses are separately encased and positioned by a cylindrical mounting, the sclera, and each lens is covered by its own cornea.
The holochroal eye with thin biconvex lenses is thought to be the ancestral form (Clarkson 1975), already well developed in the late Cambrian, while the holochroal eyes with thick prismatic lenses and the schizochroal eyes appear in post-Cambrian times. Any close similarities between trilobites' and modern arthropods' eyes is more likely to exist
Structure of the ommatidia in the compound eye of modern arthropods. Parts (a) and (b) are schematic sections through the ommatidia of the firefly. Pigment migration distinguishes the light-adapted condition (a) from the dark-adapted condition (b) (adapted from Horridgc 1969). Part (c) represents the structure of the ommatidia of the ant (adapted from Snodgrass 1952). The two types of eye are distinguished by the ptesence of short and long thabdoms respectively.
in the ancestral form of holochroal eye, where the two branches of arthropod evolution were phylogenetically closer. As we shall see, there are in fact forms in the holochroal eye that suggest a superposition-type organization. On the other hand, there is no reason to suspect that the appositiontype was not already present in trilobites. The schizochroal eye is externally organized very much in the same way as the latter. The Phacopida, however, the principal possessors of the schizochroal eyes, have evolved a sophisticated lens structure apt to correct the optical defects of thick lenses. Identical structures are not known to exist in modern arthropods, perhaps since they developed when trilobites were already a well-separated stock. However, the trilobite eye studies described in this section prompted recognition that similar morphologies and Structures observed among the dioptric apparatuses of several living arthropods serve an analogous function, a testimonial of convergent evolution aimed at satisfying a seemingly universal striving toward optimization of vision.
Most of the photographs in this section originate from negatives kindly loaned by Dr. E. N. K. Clarkson of the Grant Institute of Geology, University of Edinburgh, Scotland, who has in recent years carried out comprehensive studies of the visual apparatus of trilobites. The author has extensively collaborated with Dr. Clarkson on the study of the optical functions of several types of lens structures, and some of the discoveries emerging from this work will be described here.
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