Ciliated ducts plotted based on cilia-to-lumen ratio h / H and lumen diameter H and classified by their functions and phyla (Extended Data Fig. 6 ). Specific duct examples are illustrated by schematics. Triangles represent systems whose primary functions are thought to be (ultra-)filtration, excretion or pressure generation. Circles denote systems that transport or mix food particles, liquids or cells. Squares represent systems whose functions are at present unknown or disputed. Symbols with solid boundaries indicate perpendicular ciliation whereas symbols with a central dot have longitudinal ciliation. Symbol colour indicates phylogenetic clades listed in the colour bar. The dotted grey line indicates the h / H value identified by machine learning, which divides systems with a filtration function from systems with a bulk transport function. Power-law fits for systems with bulk transport and filtration functions are shown as dashed red lines. Their coefficients are shown in the inset.
