Figure 2

Evolutionary hypothesis for the origin and loss of appendages in the Protostomia. Based on the model shown in Fig. 1F, the common ancestor of Lophotrochozoa and Ecdysozoa, termed Urprotostomia [6], was parasegmented and had parasegmental appendages. No change of this ancestral condition is required in the lophotrochan lineage; the ancestral lophotrochozoan ("Urlophotrochozoon") is virtually identical in body organization to Urprotostomia. In the ecdysozoan lineage the processes of re-segmentation and appendage primordium expansion were evolved. This likely happened before the split of all extant ecdysozoans (arrow), based on the arthropod-like expression of en in the appendages of onychophorans [12]; the ancestral ecdysozoan ("Urecdysozoon") thus had already an adult body organization consisting of segments and segmental appendages. The limb-less forms in both Lophotrochozoa and Ecdysozoa must then be derived from limb-bearing forms by secondary loss of appendages (denoted by the black bars).