ABSTRACT: Geological, paleo-oceanographic and paleoclimatological studies have independently demonstrated that the volume of preserved marine sediments, eustatic sea level, the depth to which the sea was oxygenated, the concentration of atmospheric oxygenand carbon dioxide and the amount of terrestrial rainfall fluctuated in a 400 million yr cycle over Phanerozoic time. These data are used to develop the hypothesis that the level of oceanic turbidity co-varied with these changes and that turbidity wasthe motor, plate tectonics the tempo and perception the mode of evolution in Phanerozoic seas. This hypothesis predicts that during turbidity maxima (Silurian-Devonian, Jurassic-Tertiary), animals with non-visual foraging and predator-avoidance mechanismsand adaptations to marine pelagic, freshwater and terrestrial habits should diversify. During turbidity minima (late-Precambrian-Cambrian, late-Carboniferous-Triassic), animals with visual sensory modalities and benthic habitat specializations shoulddiversify. At the boundary of these environmental conditions, alternating mass extinctions of these adaptive characters and the taxa possessing them should occur. Patterns in the evolution of perceptual modalities, foraging strategies, predator avoidancemechanisms, habitat specializations and inferred aspects of behavioral ecology of aquatic arthropods, i.e. adaptive characters that are less sensitive to instantaneous changes in diversity than are the taxa that carried them, are used to test thesepredictions. Trilobite diversification and extinction; the adaptive radiation of ostracods and eumalacostracan crustaceans; the iterative evolution of small, fast-moving entomostracan crustaceans; and the evolution of terrestrial chelicerates and decapodcrustaceans are all consistent with these predictions. In short, the history of aquatic evolution in Phanerozoic seas can be told in the evolution of perception. Data as disparate as the diversification and extinction of marine plants, cephalopod mollusksand fish are also consistent with predictions deduced from this hypothesis.