Figure 7
The phylotypic stage in free-living flatworms. (A-F), schematic representations of embryos of the major groups of free-living flatworms at the moment of organ specification (stage 6 according to [67]). Regardless of their particular early steps of embryogenesis, free-living flatworm embryos exhibit the greatest similarity at this point of development, being also the stage at which the basic body plan is defined. In all groups, an anterior neuropile and a mid-posterior ventral pharynx start to differentiate, whereas the abundant yolk (either external or internal) adopts a dorsal position. In macrostomids and neoophoran groups, the definitive epidermis differentiates from the ventral side of the embryo to the most dorsal regions, superseding the yolk mantle or the hull membranes, respectively. These similarities led some authors to propose this time point as the phylotypic stage of Platyhelminthes [75], a concept that we extend in this work to other groups of free-living flatworms not previously considered. In all schemes, anterior is to the left and ventral to the bottom. Drawings are not to scale.