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Parazoa

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Parazoa
Temporal range: Ediacaran - Recent
Scientific classification
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The Parazoa are an ancestral subkingdom of animals, literally translated as "beside the animals".

Description

Parazoans differ from their choanoflagellate ancestors in that they are not microscopic and have differentiated cells. However, they are an outgroup of the animal phylogenetic tree being that they do not have tissues or organs. The only surviving parazoans are the sponges, which belong to the phylum Porifera, and the Trichoplax in the phylum Placozoa.

Parazoa display no body symmetry (are asymmetrical); all other animal groups display some sort of symmetry. There are currently 5000 species, 150 of which are freshwater. Larvae are planktonic and adults are sessile.

Cladistics

The Parazoa-Eumetazoa split has been estimated at 940 million years ago.[1]

The parazoa group is now considered paraphyletic.[2] It is not included in most modern cladistic analyses. When referenced, it is sometimes considered an equivalent to Porifera.[3]

Some authors include Placozoa,[4] a phylum that consists of only one species, Trichoplax adhaerens, in the division, but they are also sometimes placed in the subkingdom Agnotozoa.

References

  1. ^ Nikoh N, Iwabe N, Kuma K et al. (July 1997). "An estimate of divergence time of Parazoa and Eumetazoa and that of Cephalochordata and Vertebrata by aldolase and triose phosphate isomerase clocks". J. Mol. Evol. 45 (1): 97–106. PMID 9211740. doi:10.1007/PL00006208. Retrieved 2009-03-20. 
  2. ^ "PARAZOA TAXONOMY". Retrieved 2009-03-20. 
  3. ^ "www.ncbi.nlm.nih.gov". Retrieved 2009-03-20. 
  4. ^ "Reviews glossary". Retrieved 2009-03-20. 

External links


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