Fig. 1

Continuity or discontinuity of antero-posterior (AP) polarity along the main body axis of different metazoans. a In most metazoans, the AP polarity of the main body is the same throughout the whole length of the body. b In arthropods, serially repeated body units (Segments: blue arrows) have the same polarity as the rest of the body but with positional values reset at each boundary between them. c The same pattern emerges in the case of asexual reproduction by transversal fission. However, in asexual reproduction by (posterior) budding, as in the acoel Convolutriloba retrogemma (d), the parent and the backwards-growing bud have opposite AP directional polarity