Article: Schizochroal eyes and vision of some Silurian acastid trilobites

The eyes of some British Silurian Acastinae are described and analysed functionally. Techniques are given for measuring the size and spatial arrangement of the lenses of schizochroal eyes and the angular bearings of their lens-axes, from which may be inferred the angular range of vision of the whole eye and the relative visual acuity in different directions. In the post-larval ontogeny of the eye of Acaste downingiae (Salter) the lens-number increases with size but the angular extent of the visual field remains constant. Two eye-variants are distinguished and attributed tentatively to sexual dimorphism. Comparisons are made with the adult eyes of Acaste downingiae macrops (Salter) and Acastoides constricta (Salter).Variation of lens-size and strong differentiation between the relative acuity in the horizontal and vertical directions of vision are perhaps the most significant features of the eyes of Acastinae, and the latter is interpreted as indicative of movement-perceiving eyes. Analogies are made with the compound eyes of recent arthropods, and the ecological significance of the work is briefly discussed.