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Platyzoa

(Infrakingdom)

Overview

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The Platyzoa () are a group of protostome animals proposed by Thomas Cavalier-Smith in 1998. Cavalier-Smith included in Platyzoa the Phylum Platyhelminthes or flatworms, and a new phylum, Acanthognatha, into which he gathered several previously described phyla of microscopic animals. Subsequent studies have supported Platyzoa as a clade,[1] a monophyletic group of organisms with a common ancestor, while differing on the phyla included and on relationships within Platyzoa.

Phyla

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One current scheme places the following traditional phyla in Platyzoa:

Characteristics

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The Platyhelminthes and Gastrotricha are acoelomate. The other phyla have a pseudocoel, and share characteristics such as the structure of their jaws and pharynx, although these have been secondarily lost in the parasitic Acanthocephala. They form a monophyletic subgroup called the Gnathifera.

The name "Platyzoa" is used because most members are flat, though rotifers are not.[2]

Classification

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The Platyzoa are close relatives of the Lophotrochozoa, and are sometimes included in that group. Together the two make up the Spiralia.

he Platyzoa () are a group of protostome animals proposed by Thomas Cavalier-Smith in 1998. Cavalier-Smith included in Platyzoa the Phylum Platyhelminthes or flatworms, and a new phylum, Acanthognatha, into which he gathered several previously described phyla of microscopic animals. Subsequent studies have supported Platyzoa as a clade,[1] a monophyletic group of organisms with a common ancestor, while differing on the phyla included and on relationships within Platyzoa.

Phyla

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One current scheme places the following traditional phyla in Platyzoa:

  • Platyhelminthes
  • Gastrotricha
  • Gnathifera
    • Rotifera
    • Acanthocephala
    • Gnathostomulida
    • Micrognathozoa
    • Cycliophora

Characteristics

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The Platyhelminthes and Gastrotricha are acoelomate. The other phyla have a pseudocoel, and share characteristics such as the structure of their jaws and pharynx, although these have been secondarily lost in the parasitic Acanthocephala. They form a monophyletic subgroup called the Gnathifera.

The name "Platyzoa" is used because most members are flat, though rotifers are not.[2]

Classification

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The Platyzoa are close relatives of the Lophotrochozoa, and are sometimes included in that group. Together the two make up the Spiralia.

References

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  1. ^ Passamaneck Y, Halanych KM (July 2006). "Lophotrochozoan phylogeny assessed with LSU and SSU data: evidence of lophophorate polyphyly". Mol. Phylogenet. Evol. 40 (1): 20?8. doi:10.1016/j.ympev.2006.02.001. PMID 16556507. http://linkinghub.elsevier.com/retrieve/pii/S1055-7903(06)00051-0. 
  2. ^ "Explanations.html". http://science.kennesaw.edu/~jdirnber/InvertZoo/Tree/Explanations.html. Retrieved 2009-06-28. 

Taxonomy

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The Infrakingdom Platyzoa is a member of the Branch Protostomia. Here is the complete "parentage" of Platyzoa:

  • Domain: Eukaryota Whittaker & Margulis,1978 - eukaryotes
    • Kingdom: Animalia C. Linnaeus, 1758 - animals
      • Subkingdom: Bilateria (Hatschek, 1888) Cavalier-Smith, 1983 - bilaterians
        • Branch: Protostomia Grobben, 1908 - protostomes
          • Infrakingdom: Platyzoa T. Cavalier-Smith, 1998

The Infrakingdom Platyzoa is further organized into finer groupings including:

Phyla

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Acanthocephala

Acanthocephala (Greek , akanthos, thorn + ?efa??, kephale, head) is a phylum of parasitic worms known as acanthocephales, thorny-headed worms, or spiny-headed worms, characterized by the presence of an evertable proboscis, armed with spines, which it uses to pierce and hold the gut wall of its host. Acanthocephalans typically have complex life cycles, involving a number of hosts, including invertebrates, fishes, amphibians, birds, and mammals. About 1150 species have been described.[citation needed] [more]

Acoelomorpha

[more]

Cycliophora

Symbion is the name of a genus of aquatic animals, less than ? mm wide, found living attached to the bodies of cold-water lobsters. They have sac-like bodies, and three distinctly different forms in different parts of their two-stage life-cycle. They appear so different from other animals that they were assigned their own, new phylum Cycliophora shortly after they were discovered in 1995. This was the first new phylum of multicelled organism to be discovered since the Loricifera in 1983. [more]

Gastrotricha

The gastrotrichs (from Greek ?ast??, gaster ["stomach"], and ????, thrix ["hair"]), often called hairy backs, are a phylum of microscopic (0.06-3.0 mm) animals abundant in fresh water and marine environments. Most fresh water species are part of the periphyton and benthos. Marine species are found mostly interstitially in between sediment particles, while terrestrial species live in the water films around grains of soil. [more]

Gnathostomulida

Gnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine animals. They inhabit sand and mud beneath shallow coastal waters and can survive in relatively anoxic environments. They were first recognised and described in 1956. [more]

Platyhelminthes

The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes (from the Greek p?at?, platy, meaning "flat" and ?????? (root: ??????-), helminth-, meaning worm) are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals. Unlike other bilaterians, they have no body cavity, and no specialized circulatory and respiratory organs, which restricts them to flattened shapes that allow oxygen and nutrients to pass through their bodies by diffusion. [more]

Rotifera

The rotifers (Rotifera, commonly called wheel animals) make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703. Most rotifers are around 0.1?0.5 mm long (although their size can range from 50 ?m to over 2 millimeters), and are common in freshwater environments throughout the world with a few saltwater species; for example, those of genus . Some rotifers are free swimming and truly planktonic, others move by inchworming along a substrate, and some are sessile, living inside tubes or gelatinous holdfasts that are attached to a substrate. About 25 species are colonial (e.g., Sinantherina semibullata), either sessile or planktonic. Rotifers are an important part of the freshwater zooplankton, being a major foodsource and with many species also contributing to the decomposition of soil organic matter. Most species of the rotifers are cosmopolitan, but there are also some endemic species, like Cephalodella vittata to Lake Baikal. Recent barcoding evidence, however, suggests that some 'cosmopolitan' species, such as Brachionus plicatilis, B. calyciflorus, Lecane bulla, among others, are actually species complexes. [more]

At least 2,525 species and subspecies belong to the Phylum Rotifera.

More info about the Phylum Rotifera may be found here.

References

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  1. ^ Passamaneck Y, Halanych KM (July 2006). "Lophotrochozoan phylogeny assessed with LSU and SSU data: evidence of lophophorate polyphyly". Mol. Phylogenet. Evol. 40 (1): 20?8. doi:10.1016/j.ympev.2006.02.001. PMID 16556507. http://linkinghub.elsevier.com/retrieve/pii/S1055-7903(06)00051-0. 
  2. ^ "Explanations.html". http://science.kennesaw.edu/~jdirnber/InvertZoo/Tree/Explanations.html. Retrieved 2009-06-28. 

Sources

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Last Revised: August 24, 2012
2012/08/24 12:57:49