Fig. 6. The stiffness differential in the body column is essential for locomotion through the somersault.

(A) The extracellular matrix was perturbed locally in Hydra polyps using a partial cut (nick), to abolish the stiffness differential. The graph shows the average number of somersault events per Hydra with nicks at the shoulder or the body column. The measurements were recorded 6 h after the manipulation. (B) The extracellular matrix was perturbed globally using the chemical disruption of collagens by treatment with 10 mmol l⁻¹ dipyridyl. The average number of somersault events per Hydra was reduced upon treatment with dipyridyl for 36 h, and no events were observed after treatment with dipyridyl for 72 h. (C) The stiffness differential was perturbed by disrupting the extracellular matrix with a combination of dipyridyl treatment and a partial nick. As shown in the graph, this led to a reduction in the average number of somersault events observed compared with that for animals not treated with dipyridyl. In both cases, measurements were performed 36 h after treatment. For all the graphs, the error bars represent s.e.m., and the significance values are calculated using two-tailed Student’s t-test (*P<0.05, **P<0.005; n=20 polyps per experiment, n=3 experiments). See also Table S4.