Ecdysozoan model systems and the reconstruction of ancestral characters. (A) The Ecdysozoa (molting animals) comprises three main lineages, namely Scalidophora, Nematoida, and Pan-Arthropoda. C. elegans and D. melanogaster, the two most important invertebrate model systems, belong to Nematoida and Pan-Arthropoda, respectively. The study of a representative of the third main ecdysozoan lineage, the Scalidophora, and in particular of the marine Priapulida, will shed light on ancestral character states present at the base of the Ecdysozoa, and thus on the evolutionary events that occurred in the lineages leading to nematodes and arthropods. Phylogenetic relationships are according to . (B) Adult priapulid of the species Priapulus caudatus. Priapulids are sausage-shaped annulated worms, with an anterior introvert (in), a terminal mouth (mo), a trunk (tr), and a posterior caudal appendage (ca).