Figure 2From: Parasegmental appendage allocation in annelids and arthropods and the homology of parapodia and arthropodiaEvolutionary 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).Back to article page