bilaterian

The nature of the first bilaterian is a matter of debate.

Thus chaetognaths may be a useful model for the ancestral bilaterian.

The description of Vernanimalcula as bilaterian has been strongly challenged.

This has led to the suggestion that the gene originally served a neurogenetic function in the ancestral bilaterian.

Even if it is not a protostome, it is widely accepted as a bilaterian.

Functional considerations suggest that the surface of the bilaterian was probably covered with cilia, which it could have used for locomotion or feeding.

Although some paleontologists dispute its classification as a mollusc, it is generally accepted as being at least a bilaterian.

Professor Dunn said that the work is 'as distant as an animal can be in bilateria and still be a bilaterian.

Kimberella is a monospecific genus of bilaterian known only from rocks of the Ediacaran period.

Xenoturbella, a bilaterian whose only well-defined organ is a statocyst, was originally classified as a "primitive turbellarian".

There are earlier, controversial fossils: Vernanimalcula has been interpreted as a bilaterian, but may simply represent a fortuitously infilled bubble.

Earlier fossils are controversial; the fossil Vernanimalcula may be the earliest known bilaterian, but may also represent an infilled bubble.

Fossil remains of diverse bilaterian forms from the Lower Cambrian have been obtained from many other regions of the globe as well.

Microschedia is an enigmatic fossil bilaterian known from four specimens from Lower Cambrian deposits in Morocco.

However, as biologists' understanding of the major bilaterian lineages increases, it is beginning to appear that some of these features may have evolved independently in each lineage.

Macrostomum lignano, like all other flatworms, is an unsegmented, soft-bodied bilaterian without body cavity, and no specialized circulatory or respiratory organs.

The eyes of vertebrates usually contain cilliary cells with c-opsins, and (bilaterian) invertebrates have rhabdomeric cells in the eye with r-opsins.

They saw that the tracks had been made by a bilaterian, or an animal with bilateral symmetry, with a front and back as well as a top and bottom, unlike corals and sponges.

More recent estimates are compatible with an Ediacaran bilaterian, although it is possible, especially if early bilaterians were small, that the bilateria had a long cryptic history before they left any evidence in the fossil record.

If these features were of key importance to the evolution of large size, it is possible that bilaterian lineages each stumbled upon them independently, and perhaps even co-opted the same underlying genetic machinery from a different role.

Living Echinoderms (sea stars, sea urchins, sea cucumbers, etc.) 'look' radially symmetrical (like wheels) rather than bilaterian, but their larvae exhibit bilateral symmetry and some of the earliest echinoderms may have been bilaterally symmetrical.

It is at least as easy to assume that they evolved segmented bodies independently as it is to assume that the ancestral protostome or bilaterian was segmented and that segmentation disappeared in many descendant phyla.

Whilst the gene was traditionally understood to have served a role in segment polarization in the ancestral bilaterian, its association with shell formation in molluscs has produced an alternative hypothesis: that the ancestral role was associated with mineralization.

Taken together, sceptics doubt that the available evidence is enough to reliably identify Kimberella as a mollusc or near-mollusc, and suggest that it is presumptuous to call it anything more than a "possible" mollusc, or even just a "probable bilaterian".