Figure 4.

Evolution of the mucociliary sole. (a) Body fossil of Dickinsonia costata associated with a series of feeding traces, reproduced with permission from [3]. Numbers delineate the order of their formation in relation to the body fossil at the end of the series of traces (trace #3 made last). Scale bar, 2 cm. (b) Cell types of the placozoan Trochoplax, reproduced with permission from [103]. The thick ventral body surface is composed of ventral epithelial cells (vci), each bearing a cilium and multiple microvilli; lipophil cells (lc); and gland cells (gc), distinguished by their contents of secretory granules and prevalence near the margin. Dorsal epithelial cells (dec) constitute the dorsal surface. In between the dorsal epithelium and ventral plate are fibre cells (fc) with branching processes that contact each of the other cell types. cc, crystal cell; dci, dorsal ciliated cell; gc, gland cell. (c) A hypothetical gastraea-like ancestor with mucociliary sole. In these animals, mucocytes, zymocytes and serotonergic cells (brown) and motile ciliary cells (red) are located around the gastric opening. Contractile cells with ring-shaped fibres, excretory cells and enterocytes (all green) populate the gastric cavity. Mechanosensory cells (yellow) and contractile epithelial cells (orange) are hypothesised for a primordial nerve net. Blue arrows indicate direction of water flow across the mucociliary sole and into the gastric cavity. (d) Hypothetical diversification scheme for mucociliary cell types. (e) Hypothetical diversification scheme for endochoanocytes. (f) Hypothetical diversification scheme for ectochoanocytes into nerve net cells.