1. The peristomial membrane (PM) surrounds the mouth of regular sea-urchins and links the test to the masticatory apparatus or lantern. The PM of Paracentrolus lividus was tested mechanically to determine its basic tensile properties. 2. When subjected to vertical deformation the PM exhibits non-linear viscoelasticity, showing firstly low then high stiffness. 3. The PM can act as an elastic storage element which re-erects the lantern passively after lateral tilting. 4. The stiffness of the PM is reduced by the anaesthetic propylene phenoxetol and increased by acetylcholine or a high K+ concentration. 5. It is concluded that the PM contains mutable collagenous tissue whose tensile properties are under neural control.