(a) Reconstruction of the ventral anatomy of Agnostus pisiformis (Wahlenbetg), after scanning electron microscope observations of unusually well preserved enrolled larvae, (b) Dorsal view of adult individual (holaspid) of the same agnostid, typically only a few millimeters long, (c) Side view of enrolled individual. (From Miiller and Walossek 1987, by permission of Universitetsforlaget, Blinolern, Oslo.)
Although substantially different from the structure of the multisegmented, or polymeric, trilobites, the agnostids have customarily been regarded as trilobites. However, the new detailed information on the structure of agnostid appendages that is summarized in the reconstruction of figure 6 reveals several features that are shared with primitive crustaceans, raising questions as to whether the agnostids are indeed trilobites at all.
After the beautifully illustrated report by Miiller and Walossek appeared, I remembered collecting agnostids from the same area of Vastergotland, on a very hot summer day, accompanied by my friend and mentor Jan Bergstrom. It was a pleasant surprise to rediscover some of this material, and the result is shown in plate 13. The accomplishment of the above authors in revealing the inner structures encased in these miniature shields appears most impressive indeed.
In order to conclude this section on a light note, plate 14 shows what polymeric trilobites actually may have looked like to the casual observer. The two creatures shown, one in the normal crawling posture, the other helplessly overturned, were photographed by the author on the patio of a beautiful resort in the mountains overlooking Oslo, where the International Conference on Trilobites was held in July, 1973. We are dealing here with reconstructions of Olenoides serratus (Rominger), prepared at the Paleontological Museum, University of Oslo, and displayed for the enjoyment of the convening paleontologists. In spite of their appearance in the plate, trilobites were not light-emitting animals. The picture is simply a print from a color slide and is therefore what is usually regarded as a negative. These trilobites possessed another pair of sensory appendages, posteriorly located and called cerci. They seemed to enjoy the midsummer Norwegian sun.
Not only the trilobites developed highly organized visual organs, but some of the recently discovered properties of trilobite's eye lenses represent an all-time feat of function optimization. We are confronted here with a very successful scheme of eye structure: the composite or compound eye, made of arrays of separate optical elements, the ommatidia, pointing in slightly diverging directions and each performing an identical function. A network of photoreceptors and neurons translates the optical stimuli into an image perception. Evidence of the success of such a scheme is widespread experience, since the eyes of insects and crustaceans, in fact of most arthopods, still follow a design closely related to that developed by trilobites.
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