Fig. 4. Analysis of the kinetics of the contraction of incurrent (A,B) and excurrent (C,D) canals shows that waves of contraction propagate along and across canals. (A,C) Sponges grown as sandwich preparations were stimulated by addition of inedible ink. Measurements of incurrent (A) and excurrent (C) canal diameters over time are plotted in (B) and (D), respectively. (B,D) In B the wave of contraction propagated between sites 1 and 2 (100 µm apart) in 300 s, a rate of 0.33 µm s^–1. The wave of contraction reached site 3 with a delay of 150 s. Cells crawling through the mesohyl (indicated by white and black stars on A and B) arrest movement for approximately 10 min (B, white arrows) while the wave of contraction passes. In D the wave of contraction propagated between sites 3 and 4 (300 µm apart) in 940 s, rate of 0.32 µm s^–1. in, incurrent; ex, excurrent; arrows indicate direction of water flow in the canal. Scale bars, 100 µm (A); 300 µm (C). (Movie 3 in supplementary material.)