SEM view of the eye of a young specimen of Sphaerophthalmus alatus
(Boeck) (x203), Uppet Cambrian, Sweden. (Negative loaned by E. N. K. Clarkson; Clarkson 1973b.) The corneal membrane covers the entite visual surface; however, the swellings due to the underlying lenslets are clearly visible.
views of the eye of Scutellum (Paralejurus) campaniferum (Beyrich), a Devonian trilobite from Bohemia. The toroidal shape of the combined surface of both eyes could evidently span a visual field close to 4lt radians. Here the corneal membrane is missing and the remarkable packing arrangement of the lens elements is clearly exposed. On a small scale, the lenses are arranged in a hexagonal pattern, much like the cells of a beehive. On a broader scale, a pattern of intersecting logarithmic spirals emerges, much like the arrangement of florets in a giant sunflower. The properties of the logarithmic (or equiangular) spiral have fascinated naturalists and poets for centuries (D'Arcy-Thompson 1942). It arises in nature whenever growth (linear expansion) combines with the necessity of preserving circular forms (rotational symmetry). The growth of the trilobite eye, most noticeably of the holochroal variety, was dominated by this theme, as Clarkson (1975) has pointed out. Just as interesting as the regularities of these spiraling patterns are the imperfections and mistakes in the packing program and the steps that have been taken to correct them. Various dislocations in the pattern have been repaired while still preserving the major symmetries.
In plate 18 we see an SEM picture of a portion of the eye of Paralejurus brongniarti (Barrande), also from the Devonian period of Bohemia. Although the pattern arrangement is hexagonal, the exposed terminations of the lens elements appear quite spherical. Plates 19a and 19b show different views of the eyes of Pricyclopyge binodosa (Salter), from the Ordovician period of Bohemia. The preservation of these remarkable eyes is peculiar since only the omma-tidial lens framework is preserved as an empty dome-shaped beehive. This indicates that between adjacent units there might have been a wall, which in this case is the only preserved component. The cross section of the lens assembly is exposed in cuts and holes, thus giving a measure of its thickness. This ttilobite had immense eyes in relation to the size of its body. The eyes were placed on the sides of the cephalon, so as to extend to the ventral region, and would almost touch each other in front. In related forms, the eyes actually merged into one uninterrupted visual surface which looked like a panoramic dome. The arrangement of the lenses in the eye shown in plate 20 is unusual. We see two generative growth zones, each of them initiating a spiraling pattern in opposite directions. The two diverging patterns come together along a straight line in the central region of the eye. Finally, plates 21a and 21b show two SEM views of a cross section through the visual surface of Asaphus raniceps Dalman (Clarkson 1973a). The words of Lindstrom (1901) in describing this structure are most appropriate here: the lenses "are columnar prisms, like the pillars of basalt." These are the structures made of oriented calcite mentioned in connection with the description of figure 8. As noted by Clarkson, the symmetrically radiating arrangement of the ommatidial prisms in this trilobite reminds one of the arrangement of the superposition eye of modern arthropods. For illustration of this point, figure 9 shows a cross section through the eye of Asaphus compared with that of a night moth, Deilephila elpenor (in the light adapted condition, in order to show the crystalline cones) (Hoglund 1966). The actual photographs of a thin film section through the Asaphus'eye is shown in plate 22 and a magnified detail of the array of hexagonal prisms that make up its visual surface is shown in plate 23. Each element is terminated by an essentially flat surface on the outside of the eye and by a convex spherical surface on the inside. These are disproportionally thick plano-convex lenses. As I mentioned earlier, the lens elements of trilobites' eyes were made of single-oriented calcite crystals. This is spectacularly demonstrated in plate 23, where the single-crystal nature and crystal orientation of each element are clearly revealed by the array of cleavage planes, which cross the prisms at a constant angle to the axis of each element. From the symmetry of the cleavage pattern, it can be deduced that the optic axis (the f-axis) of the calcite crystals is aligned in all cases with the lens element axis. The shape and proportions of the globular visual surface of the Asaphus' eye give us some clues about the inner workings of the holochroal eye. Judging from the profile and composition of the lens elements, and assuming that the eye cavity was filled with a medium that had refractive index not very different from that of water, one can conclude that each lens element must
Two views of the same eye of Scutellum (Paralejurus)
campaniferum (B eyri ch) (xl5). (Negative loaned by E. N. K. Clatkson.) This Devonian trilobite from Bohemia could cover an almost spherical visual field.
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