Hohchroal Eyes

A schematic view of the range of optical structures in this type of eye is shown in figure 8, adapted from Lindstrom (1901). In addition to the basic hexagonal prism design, other lens contours were present, including square prisms. The optical behavior of these elements, while seemingly obvious in the simple biconvex lenses of Sphaerophthalmus, conceals a surprising feat, which will become apparent in the case of the long prisms of Asaphus and Illaenus in particular. Such prisms were made of single calcite crystals (Clarkson 1973a), and, to offset the strong birefringence of calcite, the crystals were oriented so that the optic axis always pointed in a direction normal to the visual surface. Only along this axis, in fact, does calcite behave as an isotropic medium. A precise understanding of the function of this unusual apparatus—whether as a light guide or as a focusing device—is still lacking. Furthermore, it is also not known whether or not crystalline cone and fiber optics accompanied the lens that is preserved. Conceivably, the long prismatic bodies could have encompassed both functions of the lens-crystalline cone assembly. Possible schemes of vision based on this kind of eye structure will be discussed further. The number of individual optical elements in the holochroal eye could vary from approximately one hundred to more than fifteen thousand in a single eye, a range not very different from that found in insects. The actual size of the eye, however, often exceeds that of modern arthropods.

Perhaps nothing better than the photographic record can convey a feeling for these structures and, at the same time, show the overall shape of the eye and its visual field. Plates 15 and 16 show scanning electron microscope (SEM) pictures (Clarkson 1973b) of some of the ancestral holochroal eyes from the olenid trilobites of the Late Cambrian period of Sweden. In plate 15, the eye of Ctenopyge (Meso ctenopyge) tumida is seen partly covered by the corneal membrane. Where this is still present, the swelling caused by the presence of the underlying lenslets is apparent. Where the membrane is removed, only the lensar pits remain of the visual surface. The thin lens profile can be made out by comparing the two regions. Plate 16 shows an intact eye of Sphaerophthalmus alatus. The external similarities of these primitive eye forms to those of some modern insects (for example, the ant) is quite remarkable. Plate 17 contains two

figure 8

Cross-sectional and frontal views of the visual surface in several holochroal eyes of trilobites. (a) Sphaerophthalmus alatus (Boeck), U. Cambrian, Sweden (x80); (b) Cyrtometopus clavifrons (Dalman), Ordovician, Sweden (x50); (c) Illaenus chiron Holm, Ordovician, Sweden, (x50). Adapted from Lindstrom 1901.

figure 8

Cross-sectional and frontal views of the visual surface in several holochroal eyes of trilobites. (a) Sphaerophthalmus alatus (Boeck), U. Cambrian, Sweden (x80); (b) Cyrtometopus clavifrons (Dalman), Ordovician, Sweden (x50); (c) Illaenus chiron Holm, Ordovician, Sweden, (x50). Adapted from Lindstrom 1901.

Scanning electron microscope view of the eye of Ctenopyge (Mesoctenopyge) tumida Westergard (x 117). Upper Cambrian, Sweden. (Negative loaned by E. N. K. Clarkson; Clarkson 1973b.) The visual surface is partially covered by the corneal membrane.

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