The peristomial membrane (PM) is the area of flexible body wall which surrounds the mouth of regular sea-urchins. In cidaroid sea-urchins, like Stylocidaris. affnis (Phil.), it contains an extensive endoskeleton of overlapping plates. This paper describes the basic mechanical properties of the PM of S. affinis and compares them with those of the PM of the previously studied echinacean euechinoid Paracentrotus lividus. When subjected to vertical deformation, the PM of S. affinis generates J-shaped tension-deformation curves with an inflexion point at a deformation corresponding to about 3% of the perignathic girdle diameter. It is considerably stiffer than the P. lividus PM and, unlike the latter, it shows asymmetrical properties, i.e. greater resistance to aboral flexion (retraction) than to oral flexion (protraction). In interambulacral regions the junction between the PM endoskeleton and test of S. affinis takes the form of a wide ligament-filled gap which acts as a compliant hinge. In marked contrast to the echinacean PM, that of S. affinis provides little evidence for the presence of mutable collagenous tissue (MCT). The cidaroid PM is interpreted as a design which maximizes mechanical defence whilst retaining the capacity for some flexibility. A possible correlation between the lantern systems and feeding habits of sea-urchins is discussed.