We measured coelomic pressure in sea urchins to determine whether it was high enough to support a pneu hypothesis of growth. In Strongylocentrotus purpuratus the pressure was found to fluctuate rhythmically about a mean of -8 Pa, and was negative for 70% of the time. This is at variance with the theoretically required positive pressures of the pneu hypothesis. Furthermore, there were no sustained significant differences between the pressure patterns of fed and starved urchins, presumed to be growing and not growing, respectively. The rhythmical fluctuations in pressure were caused by movements of the lantern which changed the curvature and tension of the peristomial membrane. We developed a mathematical and morphological model relating lantern movements, membrane tension, and pressure, that correctly predicts the magnitude of the fluctuations. Pressures predicted by the model depend also on coelomic volume changes. In Lytechinus variegatus simultaneous retraction of the podia, which causes expansion of the ampullae, resulted in an 8.8 Pa increase in coelomic pressure, relative to the pressure during simultaneous podial protraction.