Fig. 8. (A) Typical stress–strain behaviour of a slab of isolated mesoglea in the radial direction. The figure is constructed from the first load and unload cycle to avoid errors due to tissue damage and degradation during the experiment. Negative strains represent compressive loading, while positive strains indicate tension beyond the resting thickness. The sample was compressed below its resting thickness during mounting, with the result that the fibres, normally pre-strained in vivo, were slack. The joint mesoglea was removed in this preparation, so it was possible to measure the stiffness, E_m (broken line), of the bell mesoglea alone. The stiffness is the slope of a straight line fit to the data in the region between the extension and compression shoulders. The solid line shows the parallel stiffness, E_L, which includes contributions from matrix and fibres. Again, the stiffness was determined from the slope of a line fit to the straight line region near the ultimate strain, defined as the strain just before the tissue began to yield. For most jellyfish, the ultimate strain was approximately 35%, which corresponds well to the 36% radial strain observed by Gladfelter (1972) during swimming. Data for eight jellyfish tested in this manner are summarised in Table 3 (the data shown in this figure are from Jellyfish 19). Zero strain was determined by regressing from the large strain data, as discussed in the Materials and methods. Following Lillie et al. (1998), stiffnesses were determined using the loading curve rather than the unloading curve. The hysteresis in the isolated preparation is probably due in large part to water loss during the experiment. (B) Typical stress–strain behaviour of intact mesoglea. Negative strains represent compressive loading, while positive strains indicate tension beyond the native thickness. Because there was no loss of joint mesogleal tissue in this preparation, the slope of the broken line is the upper limit (as discussed in the text) of the stiffness of the joint mesoglea, E_jm, while the slope of the solid line is again the radial tensile stiffness parallel to the fibres and includes contributions from the matrix and fibres. Data for three jellyfish tested in this manner are summarised in Table 3 (data in this figure are from Jellyfish 25). As in A, zero strain was determined from a regression through the large strain data.