Developmental diversity in free-living flatworms

Flatworms (Platyhelminthes) are acoelomate, usually hermaphroditic, egg-laying bilaterians with multiciliated epithelial cells and are lacking a circulatory system, an anus and respiratory organs other than the epidermis [1]. The taxon is comprised of free-living and parasitic species, including flukes and tapeworms [2].Since long, the embryonic development of flatworms has attracted attention of embryologists and phylogeneticists alike for their assumed central position in the evolution of the Bilateria or even the Metazoa [3]. Several hypotheses have been formulated to reconstruct the transition from ciliates to acoels [4,5], from cnidarian planula larvae to acoels [6] or from ctenophores to polyclads [7], and phylogenetic relationships were explored and discussed by studying the ontogeny of flatworms [3]. Today, the affiliation of the Platyhelminthes to the Spiralia (or Lophotrochozoa), especially apparent in polyclad flatworms, is widely accepted and the problematic position of acoels and nemertodermatids, traditionally regarded as members of the Platyhelminthes (see [8] and literature therein), is now commonly seen outside this phylum, either as sister group to all other bilaterians [9], as sister group of the Gnathostomulida [10] or as members of the deuterostomes [11].Traditionally, two broad classifications were used to subdivide the Platyhelminthes. According to their lifestyle, flatworms were classified either in free-living forms (former class "Turbellaria") including some parasitic groups such as the Fecampiida, and in strictly parasitic organisms (Neodermata, Figure 1). Here, we use the term "free-living flatworms" in the turbellarian sense, i.e., to encompass all flatworms other than the Neodermata. In addition, the structure of the oocyte was used as a systematic criterion. Flatworms with entolecithal eggs - eggs that contain all yolk needed for development - are called "Archoophora", and this condition is considered primitive or plesiomorphic, while