Fig. 2: Quantitative analysis of head-regeneration abilities across the flatworm collection.

a, Cartoon of the serial head-regeneration assay (left) and representative outcomes in the indicated species. Dashed lines: amputation planes and typical regeneration outcomes (small images) and their relative frequency (number pairs; ‘Dead’ in case of no survivors) at the respective A–P position. Colours designate the head-regeneration classification scheme used throughout this study: group A (green): robust regeneration (efficient head regeneration at all A–P axis positions); group B (orange): restricted regeneration (position-dependent head-regeneration defects); group C (red): poor regeneration (no head regeneration at any A–P position). Missing images in the C. hastata panel correspond to pieces that did not regenerate a head and died before imaging (No head). b, Graphical representation of the data in the form of head frequency curves. The percentage of successful head regeneration is plotted either as a fraction of the initial (black line) or surviving (red line) fragment numbers. Dashed line, percentage of fragment survival. The Roman numerals below designate the more fine-grained head-regeneration scheme by Šivickis²³. c, Left, head frequency curves (percentage of surviving fragments) for all analysed species and colour-coded as in a. Right, number of species in each head-regeneration category. d, Unclassified regeneration defects and their relative frequencies in the indicated collection species, including bipolar double-heads (left), double-tails (centre) and mediolateral axis duplication (right). Amputation paradigms are cartooned to the left; the respective regeneration outcome in Smed is shown for reference. e, Phylogenetic overview of flatworm groups related to planarians⁴⁰ (left); amputation paradigm (centre); and representative images and quantifications of head-regeneration failures in the indicated Prolecitophora species (right). Scale bar, 1 mm unless otherwise noted.