Experiments are described which suggest that Calliactis behaves as a closed animal in its movements, although its stomodaeum appears to be widely open to the exterior.

Observations of the movements of the animal are correlated with variations of the internal pressure. It is shown that expansion is not merely ‘passive relaxation’, but is generally accompanied by a rise in pressure which is, however, much lower than that which accompanies rapid closing.

It is concluded that the fluid-muscle system of Calliactis behaves in a similar way to that of other soft-bodied invertebrates, but that the system works at a much lower pressure.