They are distributed into the following taxa: Acoela, Nemertodermatida, Catenulida, Macrostomida, Polycladida, Prolecithophora, Lecithoepitheliata, Revertospermata, Proseriata, Tricladida, Dalytyphloplanida and Kalyptorhynchia (Schockaert et al.
The following key-words were used: Platyhelminthes, Acoela, Nemertodermatida, Catenulida, Prolecithophora, Rhabdocoela, Temnocephalida, Macrostomida, Lecithoepitheliata or Revertospermata and Brazil.
Dalytyphloplanida and Catenulida are the most species-rich, with about 28% and 18% of the recorded species, respectively, followed by Acoela (15%), Prolecithophora (11%), Proseriata (10%), Kalyptorhynchia (9%), and Macrostomida (7%).
However, new information, especially from molecular phylogeny, has demonstrated that the Acoela and Nem-ertodermatida.
A phylogenetic analysis of myosin heavy chain type II sequences corroborates that Acoela and Nemerto-dermatida are basal bilaterians.
generation of new fibers by fission of existing fibers seems to be an evolutionarily conserved mechanism that also occurs in Acoela, presumably the most basal bilaterian animals (Semmler et al.
The bodywall musculature of the spiralian last common ancestor probably contained outer ring, inner longitudinal, and maybe also intermediate diagonal muscles, similar to the condition found in the most basal extant bilaterians, the Acoela (Ladurner and Rieger, 2000; Semmler ei al, 2008).