Narrowing of the blastopore and the beginning of mouth formation in the mollusc Crepidula fornicata. The actin cytoskeleton at the edge of each cell has been stained green. (From Lyons et al 2015. Spiralian gastrulation: germ layer formation, morphogenesis, and fate of the blastopore in the slipper snail Crepidula fornicata. Evodevo. doi: 10.1186/s13227-015-0019-1. Used with permission of D. Lyons.)