Figure 5. Hypotheses on the evolution of metazoan development.

Acquisition of feeding larvae shown with circles, and loss of feeding larvae shown with crosses. (a) Xenacoelomorpha as an early offshoot of the bilaterians suggested by Hejnol et al.3 Similarities with cnidarian planula may indicate an ancestral developmental stage for metazoans. (b,c) Xenacoelomorpha as deuterostomes suggested by Phillipe et al.4 (b) If Xenacoelomorpha direct development was secondarily derived, feeding larvae may have been lost independently in Ecdysozoa, Xenacoelomorpha and Chordata. (c) If Xenacoelomorpha direct development was retained from the metazoan ancestor, planktotrophic larvae with ciliary bands may have been independently acquired in the Ambulacraria and the Lophotrochozoa. Drawings of Acoelomorpha are modified from Ramachandra et al.6, those of Lophotrochozoa and Ambulacraria from Arendt et al.34, and Cnidaria from Kamm et al.35, respectively.