Fig. 6. (A) Local fields used to measure the spreading of local contractile waves over the sponge body. Field a represents an measurement area of 3 mm², field b represents 2 mm²; both are not completely filled by sponge, to record changes due to contraction; distance d between a and b is 3 mm; bar, 2.5 mm. (B) Changes in the projected areas of fields a and b during a contraction event. The maximum contraction spreads as a wave over the sponge surface, taking 4 min to traverse the 3 mm distance, a speed of 750 µm min^-1 (=12.5 µm s^-1). The contraction used for this measurement is shown in movie S4 in supplementary material.