Simplified diagram of Spirorbis spirorbis showing coiled calcareous shell with a collar (co) at opening, protruding operculum (op) and branchial crown comprising of radioles (ra) and pinnules (pi). The lower region is dissected open to show the membranous tube encompassing the embryos in the anterior abdomen, and segmentation of body. Adapted from Brinkmann and Wanninger, 2009.