WHILE CNIDARIA are considered to have radial symmetry, a new article proposes that the cnidaria had a bilaterian ancestor and may properly be placed in Bilateria.1


The paper starts out by considering hypotheses regarding the urbilaterian, the last common ancestor of all bilaterians. There are two schools of thought, one saying that this ancestor was very simple, and one saying that it already had a suite of advanced features, sometimes extending to segmentation and appendages. Due to their apparent simplicity the platyhelminthes were originally thought to be similar in structure to the urbilaterian, being acoelomates with a single digestive opening. However, with the proposal that the protostomes be split into Lophotrochozoa and Ecdysozoa, platyhelminths were categorized as lophotrochozoans that were secondarily acoelomate. This development plus deductive reasoning based upon the shared characteristics of bilaterians have given support to the complex urbilaterian theory.

However, Baguñà and co-workers previously proposed in two almost identically titled papers that two groups previously included in Platyhelminthes are actually not closely related to these but are basal bilaterians. These are Acoel2 and Nemertodermatida3. Recently Jondelius reiterated this position more forcefully.4 Neither of these groups was included in the recent study on the metazoan tree of life, acoels because they did not produce a stable result and nemertodermatids because there was not enough information.5 If these are basal bilaterians, that would lend support to the idea that the urbilaterian was a relatively simple acoelomate with a blind digestive tract. Unfortunately at this time it appears only Baguñà and Jondelius are interested in the phylogenetics of these creatures.

New bilaterian treeNow Baguñà is shaking things up some more with the suggestion that Bilateria should properly be expanded to include the Cnidaria, with the group currently known as Bilateria renamed to Triploblastica, for the three tissue layers present in these organisms. Anthozoans (sea anemones and corals) have some features suggesting bilateriality, including gene expression patterns, and have been described as “biradial”.6 Originally these were thought to be derived features, but recently anthozoans were established as basal members of Cnidaria. Combining these bilaterian indications with the supposed basal placement of the acoels and nemertodermatids, Baguñà proposes that:

Unless acoels and nemertodermatids are shown to be ancestral but simplified or just derived bilaterians, the new LCBA [last common bilaterian ancestor] leads to a smooth morphological and developmental transition from a bilateral, diploblastic planuloid to a bilateral, triploblast acoeloid, and from the latter to more complex higher bilaterians. Alternatively, bilateral symmetry may have evolved under selective pressure for improved internal circulation in a cnidarian–bilaterian ancestor, inferred to be a sessile, bilaterally symmetrical animal (Finnerty 2005). Additional internal manifestations of bilateral symmetry evolved subsequently in bilaterians. As for most proposals on the origin of bilateral organisms with directive locomotion based on the enterocoel–archicoelomate hypotheses, the main stumbling blocks for its acceptance are the undefined developmental mechanisms and the uncertain functional continuity of intermediates between a sessile ancestor and a benthic crawling descendent. This makes it more plausible that, as stated in the planuloid–acoeloid theory, bilaterality first originated in small, bottom-dwelling organisms.

In reading about cnidarians I ran across another paper showing that hydra have an aboral pore. Cnidarians are typically thought to have a single digestive opening, but Shimizu and coworkers discovered (or rediscovered, since it was first reported in 1928 and then ignored!) a tiny pore in the basal disc of hydra that is maintained by hydra matrix metalloprotease digestion of the mesoglea.7 While this pore does not normally permit fluid to pass through, the researchers observed on several occasions the expulsion of small volumes of digested material and the influx of dye. The pore is not present in sea anenomes, which have a thick pedal disc that remains in contact with the substrate. The authors hypothesize that Cnidaria and Bilateria are descended from an ancestor with a tube-like digestive tract, and that the blind sac digestive system evolved later in Anthozoa due to a sessile lifestyle rooted perpendicular to the substrate.

If this is true and if the protostome-deuterostome ancestor had amphistomic gastrulation, as I discussed in my post on deuterostome evolution, we might suspect homology between this pore and the mouth and/or anus in protostomes. We already know that there are conserved patterns of gene expression during gastrulation in cnidarians and protostomes, but the development of this pore has not been studied. More research will be required to confirm the existence this aboral pore, determine if it is a basal feature for cnidarians or evolved later, and explore the possibility of an ancestor with a tube-like digestive tract for cnidarians and bilaterians. However, Shimizu thinks that this ancestor with a tube-like digestive tract was radially symmetric, while Baguñà and Jondelius think it was bilaterally symmetric but had a blind digestive tract! I would be curious to see their reaction to this article.

I admit a fondness for the (moderately) complex urbilaterian, so I would like to see a response to these ideas. More research is required into the phylogenetic relationships of acoels and nemertodermatids so that the complex urbilaterian proponents can take their turn at the bat.

  1. Baguñà, J.; Martinez, P.; Paps, J.; Riutort, M. “Back in time: a new systematic proposal for the Bilateria.” Philosophical Transactions of the Royal Society B 2008, 363, 1481-1491. DOI:10.1098/rstb.2007.2238
  2. Ruiz-Trillo, I.; Riutort, M.; Littlewood, T. J.; Herniou, E. A.; Baguñà, J. “Acoel Flatworms: Earliest Extant Bilaterian Metazoans, not Members of Platyhelminthes.” Science 1999, 283, 1919-1923. DOI:10.1126/science.283.5409.1919
  3. Jondelius, U.; Ruiz-Trillo, I.; Baguñà, J.; Riutort, M. “The Nemertodermatida are basal bilaterians and not members of the Platyhelminthes.” Zoologica Scripta 2002, 31, 201-215. DOI:10.1046/j.1463-6409.2002.00090.x
  4. Walberg, A.; Gurini-Galleti, M.; Ahmadzadeh, A.; Jondelius, U. “Dismissal of Acoelomorpha: Acoela and Nemerotdermatida are separate early bilaterian clades.” Zoologica Scripta 2007, 36, 509-523. DOI:10.1111/j.1463-6409.2007.00295.x
  5. Dunn, C. W.; Hejnol, A.; Matus, D. Q.; Pang, K.; Browne, W. E.; Smith, S. A.; Seaver, E.; Rouse, G. W.; Obst, M.; Edgecombe, G. D.; Sorensen, M. V.; Haddock, S. H. D.; Schmidt-Rhaesa, A.; Okusu, A.; Kristensen, R. M.; Wheeler, W. C.; Martindale, M. Q.; Giribet, G. “Broad phylogenetic sampling improves resolution of the animal tree of life.” Nature advance online publication, 5 March 2008 (DOI:10.1038/nature06614).
  6. De Jong, D. M.; Hislop, N. R.; Hayward, D. C.; Reece-Hoyes, J. S.; Pontynen, P. C.; Ball, E. E.; Miller, D. J. “Components of both major axial patterning systems of the Bilateria are differentially expressed along the primary axis of a ‘radiate’ animal, the anthozoan cnidarian Acropora millepora.” Developmental Biology 2006, 298, 632-643. DOI:10.1016/j.ydbio.2006.07.034
  7. Shimizu, H.; Takaku, Y.; Zhang, X.; Fujisawa, T. “The aboral pore of hydra: evidence that the digestive tract of hydra is a tube not a sac.” Development Genes and Evolution 2007, 217, 563-568. DOI:10.1007/s00427-007-0165-0