The early Cambrian <i>Saccorhytus</i> is a non-feeding larva of a scalidophoran worm

With its bag-like appearance, spiny ornament and single opening, <i>Saccorhytus coronarius</i> is one of the most enigmatic animals of the early Cambrian Kuanchuanpu Formation (<i>ca</i> 535 Ma) and has been at the heart of debates concerning the origin of two major animal lineages: the deuterostomes and the ecdysozoans. Although Ecdysozoa is now the most plausible option, key aspects of its palaeobiology have remained elusive. I suggest here that <i>Saccorhytus</i> was the possible larval stage of one of the numerous scalidophoran worms that co-existed with it in the Kuanchuanpu biota, especially those with bilateral pairs of broad-based sclerites such as <i>Eokinorhynchus</i>. This new interpretation is mainly based on the development of extant priapulid worms (Scalidophora). It would imply that <i>Saccorhytus</i> is a non-feeding larva and that its enigmatic single aperture with a pleated structure is not a mouth but more likely the opening through which the juvenile exited. Thus, the strange anatomy of <i>Saccorhytus</i> could be explained in a simple way without involving complex evolutionary processes (e.g. simplification, miniaturization, loss of anus, etc.). Instead of being a relatively basal member of the total group Ecdysozoa, <i>Saccorhytus</i> would find a more likely position among the total group Scalidophora.