title: A metameric origin for the annelid pygidium?
abstract: BACKGROUND
  Segmented body organizations are widely represented in the animal kingdom. Whether the last common bilaterian ancestor was already segmented is intensely debated. Annelids display broad morphological diversity but many species are among the most homonomous metameric animals. The front end (prostomium) and tail piece (pygidium) of annelids are classically described as non-segmental. However, the pygidium structure and development remain poorly studied.
  
  
  RESULTS
  Using different methods of microscopy, immunolabelling and a number of molecular markers, we describe the neural and mesodermal structures of the pygidium of Platynereis dumerilii. We establish that the pygidium possesses a complicated nervous system with a nerve ring and a pair of sensory ganglia, a complex intrinsic musculature, a large terminal circular blood sinus and an unusual unpaired torus-shaped coelomic cavity. We also describe some earlier steps of pygidial development and pygidial structure of mature animals after epitokous transformation.
  
  
  CONCLUSIONS
  We describe a much more complex organization of the pygidium of P. dumerilii than previously suggested. Many of the characteristics are strikingly similar to those found in the trunk segments, opening the debate on whether the pygidium and trunk segments derive from the same ancestral metameric unit. We analyze these scenarios in the context of two classical theories on the origin of segmentation: the cyclomeric/archicoelomate concept and the colonial theory. Both theories provide possible explanations for the partial or complete homology of trunk segments and pygidium.
authors.affiliation: Department of Invertebrate Zoology, Saint-Petersburg State University/ Universitetskaya nab. 7/9, 199034, Saint-Petersburg, Russia. starunov@gmail.com.|Zoological Institute RAS/ Universitetskaya nab. 1, 199034, Saint-Petersburg, Russia. starunov@gmail.com.
authors.forename: Viktor V
authors.initials: VV
authors.lastname: Starunov
authors.affiliation: Institut Jacques Monod, CNRS/Université Paris Diderot, 15 rue Hélène Brion, 75013, Paris, France. nicolas.dray@inaf.cnrs-gif.fr.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Dray
authors.affiliation: Department of Invertebrate Zoology, Saint-Petersburg State University/ Universitetskaya nab. 7/9, 199034, Saint-Petersburg, Russia. 090189@mail.ru.
authors.forename: Elena V
authors.initials: EV
authors.lastname: Belikova
authors.affiliation: Institut Jacques Monod, CNRS/Université Paris Diderot, 15 rue Hélène Brion, 75013, Paris, France. kerner@ijm.univ-paris-diderot.fr.
authors.forename: Pierre
authors.initials: P
authors.lastname: Kerner
authors.affiliation: Institut Jacques Monod, CNRS/Université Paris Diderot, 15 rue Hélène Brion, 75013, Paris, France. vervoort.michel@ijm.univ-paris-diderot.fr.|Institut Universitaire de France, Paris, France. vervoort.michel@ijm.univ-paris-diderot.fr.
authors.forename: Michel
authors.initials: M
authors.lastname: Vervoort
authors.affiliation: Institut Jacques Monod, CNRS/Université Paris Diderot, 15 rue Hélène Brion, 75013, Paris, France. balavoine.guillaume@ijm.univ-paris-diderot.fr.
authors.forename: Guillaume
authors.initials: G
authors.lastname: Balavoine
mesh_terms: D000818:Animals; D005260:Female; D008297:Male; D008969:Molecular Sequence Data; D009024:Morphogenesis; D011077:Polychaeta / Q000033:anatomy & histology / Q000196:embryology* / Q000502:physiology; D013623:Tail / Q000196:embryology* / Q000648:ultrastructure