The evolution of the position of the mouth in the Ecdysozoa. (A) Diversity of mouth position in embryos and adults of the different ecdysozoan lineages. While in the Spiralia the mouth forms ventrally, and stays ventral in the adult, this situation is more variable in the Ecdysozoa. The mouth can form ventrally (priapulids, some nematodes, onychophorans, and most arthropods), at an anterior terminal position (most nematodes, nematomorphs, and tardigrades), or even on the dorsal site (in the pycnogonid arthropods). However, in the adults the mouth occupies an anterior terminal position, except in extant onychophorans and most arthropods (pycnogonids have a terminal mouth). While an anterior terminal mouth in the adult seems to be ancestral for the Ecdysozoa, the original embryonic position has been debated. (B) Evolutionary scenario of an ancestral ventral embryonic mouth. In this situation, independent modifications occurred in the Nematoida and Tardigrada that led to the late formation of the mouth at a terminal position. (C) Evolutionary scenario of an ancestral anterior embryonic mouth. In this situation, the embryonic mouth shifted to an anterior position at the base of the Ecdysozoa, and was secondarily reverted to a ventral site in priapulids and pan-arthropods, which is less parsimonious than assuming an ancestral ventral embryonic mouth (see B). For the sake of clarity, the ventral mouth of certain nematodes and the dorsal mouth of pycnogonids have not been considered in B or C. In A-C, the asterisk indicates the alternative branching of the Scalidophora (Priapulida, Kinorhyncha, Loricifera) together with the Nematoida to form what has been proposed as the Cycloneuralia clade. See main text for references. Drawings are not to scale.