The Parazoa are an ancestral subkingdom of animals, literally translated as "beside the animals".
[edit] 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. The only surviving parazoans are the sponges, which belong to the phylum Porifera, and one surviving species (Trichoplax adhaerens) 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.
[edit] 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.
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