Urmetazoan

Source: Wikipedia, the free encyclopedia.

The Urmetazoan is the hypothetical

collagen IV and fibrillar collagen, different cell types (as well as expanded gene and protein families), spatial regulation and a complex developmental plan, and relegated unicellular stages.[1]

Choanoflagellates

All animals are posited to have

flagellates whose cell morphology is similar to the choanocyte cells of certain sponges
.

eukaryotes
tend to have anterior flagella as well.

Hypotheses

Several different hypotheses for the animals' last common ancestor have been suggested.

  • The placula hypothesis, proposed by Otto Bütschli, holds that the last common ancestor of animals was an amorphous blob with no symmetry or axis. The center of this blob rose slightly above the silt, forming a hollow that aided feeding on the sea floor underneath. As the cavity grew deeper and deeper, the organisms resembled a thimble, with an inside and an outside.[2] This body shape is found in sponges and cnidaria. This explanation leads to the formation of the bilaterian body plan; the urbilaterian would develop its symmetry when one end of the placula became adapted for forward movement, resulting in left-right symmetry.[2]
  • The planula hypothesis, proposed by Bütschli, suggests that metazoa are derived from
    paedomorphosis, and could reproduce without passing through a sessile
    phase.
  • The bilaterogastraea hypothesis was developed by Gösta Jägersten as an adaptation of
    benthic
    adult stage. The invagination of the original gastrula stage he saw as bilaterally symmetric rather than radially symmetric.

See also

References

  1. ^ Ros-Rocher Núria, Pérez-Posada Alberto, Leger Michelle M. and Ruiz-Trillo Iñaki. 2021 The origin of animals: an ancestral reconstruction of the unicellular-to-multicellular transition Open Biol. 11:200359. 200359. http://doi.org/10.1098/rsob.200359
  2. ^
    PMID 19175291
    .
  3. ^ Haeckel, E. 1874. Die Gastraea-Theorie, die phylogenetische Classification des Thierreichs und die Homologie der Keimblätter. Jenaische Zeitschr. Naturwiss. 8:1-55.
  4. Wikidata Q54502332
    .

External links
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