An Unusual Body Plan in Bilateria: a Fractal Branching Body.

A branching body pattern with one head and many "tails", unique for recent Bilateria, was found in three species of endosymbiotic polychaetes of the family Syllidae (McIntosh, 1879; Glasby et al., 2012; Aguado et al., 2015, 2022). Among recent echinoderms, many species of crinoids, brittle stars, and holothurians have dendriform branching bodies with many distal, posterior ends of their arms, rays, and tentacles, which include the typical axial ambulacral complex. Multiple bifurcations of the posterior (distal) parts create fractal branching of the body pattern, representing a macroevolutionary transformation of the ancestral body plan in Bilateria. [ABSTRACT FROM AUTHOR]