Protostome

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Protostomes
Temporal range: Ediacaran - Recent
ArthropodMolluscaAnnelidNematodeFlatwormRotifer
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Subkingdom: Eumetazoa
Clade: ParaHoxozoa
Clade: Bilateria
Clade: Nephrozoa
(unranked): Protostomia
Grobben, 1908
Superphyla

Protostomia (

molluscs, annelids, flatworms and nematodes. They are also called schizocoelomates since schizocoely
typically occurs in them.

Together with the Deuterostomia and

three germ layers.[3]

Protostomy

first opening becomes the embryos' mouth while the anus is formed later. Also, the secondary body cavity (shown in blue) forms from tissue that splits away from the rest, instead of by folding in
from the gut walls.
See also: embryological origins of the mouth and anus

In animals at least as complex as

blastopore) which deepens to become its digestive tube (the archenteron). In the sister-clade, the deuterostomes (lit. 'second-mouth'), the original dent becomes the anus while the gut eventually tunnels through to make another opening, which forms the mouth. The protostomes (from Greek πρωτο- prōto- 'first' + στόμα stóma 'mouth') were so named because it was once believed that in all cases the embryological dent formed the mouth while the anus was formed later, at the opening made by the other end of the gut.[4][1]
It is now known that the fate of the blastopore among protostomes is extremely variable; while the evolutionary distinction between deuterostomes and protostomes remains valid, the descriptive accuracy of the name protostome is disputable.[1]

Protostome and deuterostome embryos differ in several other ways. Many protostomes (the Spiralia clade) undergo spiral cleavage during cell division instead of radial cleavage.[5] Spiral cleavage happens because the cells' division planes are angled to the polar major axis, instead of being parallel or perpendicular to it. Another difference is that secondary body cavities (coeloms) generally form by schizocoely, where the coelom forms out of a solid mass of embryonic tissue splitting away from the rest, instead of by enterocoelic pouching, where the coelom would otherwise form out of in-folded gut walls.[6]

Evolution

The common ancestor of protostomes and deuterostomes was evidently a worm-like aquatic animal of the Ediacaran. The two clades diverged about 600 million years ago. Protostomes evolved into over a million species alive today, compared to ca. 73,000 deuterostome species.[7]

Protostomes are divided into the

rotifers). A modern consensus phylogenetic tree for the protostomes is shown below.[8][9][10][11][12]
The timing of clades radiating into newer clades is given in mya (millions of years ago); less certain placements are indicated with dashed lines.[13]

Bilateria

Xenacoelomorpha

Nephrozoa
Deuterostomia

Ambulacraria

Chordata

Protostomia
Ecdysozoa
Spiralia
Gnathifera

Rotifera and allies

Chaetognatha

Platytrochozoa

Platyhelminthes and allies

Lophotrochozoa

Mollusca

Annelida and allies

550 mya
580 mya

†Kimberella

610 mya

See also

References

  1. ^ a b c Hejnol, A.; Martindale, M. Q. (2009). "The mouth, the anus, and the blastopore - open questions about questionable openings". In M. J. Telford; D. T. J. Littlewood (eds.). Animal Evolution — Genomes, Fossils, and Trees. pp. 33–40.
  2. S2CID 90795
    .
  3. PMID 19759036
    .
  4. ISBN 978-0-521-83762-0
    .
  5. PMID 9223303
    .
  6. ISBN 978-0-85229-961-6
    .
  7. ^ The Invertebrate tree of life, Giribet & Edgecombe, 2020; p.155
  8. S2CID 32169826
    .
  9. PMID 28377584
    .
  10. S2CID 205239797
    .
  11. ^ "Palaeos Metazoa: Ecdysozoa". palaeos.com. Retrieved 2017-09-02.
  12. PMID 26605063
    .
  13. PMID 18192191
    .

External links

Wikispecies has information related to Protostomia.
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