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Metazoa |--Porifera (paraphyletic?) `--+--Radiata `--Bilateria |--Deuterostomia `--Protostomia |--Ecdysozoa | |--Tardigrada | `--Arthropoda | |--Trilobita | `--Pancrustacea | |--Crustacea | `--Insecta `--Spiralia (Lophotrochozoa) |--Bryozoa |--Mollusca `--+--Annelida `--Brachiopoda
The Bilateral (and Coelomate) phyla can be divided into two groups, variously called the Schizocoela and Enterocoela, or the Protostomia and Deuterostomia. These are distinguished by embryonic cleavage patterns, the fate of the blastopore and coelom formation. The Deuterostomes are a relatively small (in terms of species and abundance) group that includes vertebrates, echinoderms, and a few assorted minor phyla. We probably consider them (and specifically the vertebrates) important mainly for anthropocentric reasons (we are deuterostomes). The Protostomes in contrast include the great majority of animal species, such as arthropods, molluscs, annelids, and many less well-known taxa.
The protostome condition is defined by a spiral and determinant cleavage in the early stages of embryo development, schizocoelous coelom formation (as the archenteron (embryonic gut) forms the coelom begins as splits within the solid mesodermal mass, and the formation of the blastopore (the original opening) into the mouth
Beginning with Aguinaldo et. al. 1997, molecular studies in metazoan phylogeny have divided the Protostomia can into two further groups, one called Ecdysozoa or moulting animals, the other the Lophotrochozoa or Spiralia (the first name is more widely used, but the second is preferable, both being the senior synonym and because it and describes the spiral larval development of these invertebrates). Although there was originally some molecular phylogeneticists and morphologists did not accept this approach, it has bene increasingly confirmed by later studies and is now the mainstream position. Indeed, over the last decade or so the emphasis has shifted from the distinction between protostomes and deuterostomes and determining which phyla go where, to that of the phylogenetic relationships of teh various phyla that make up the Lophotrochozoa/Spiralia and Ecdysozoa (along with a few problematica which may belong to either group), with the now well resolved Deuterostomes the third party, on the sidelines.
Protostomia Grobben, 1908 : slugs > bugs
Range: Fr Ediacaran
Phylogeny: Bilateria : Deuterostomia + * : Ecdysozoa or moulting animals, the other the Spiralia
Characters: Blastopore becomes mouth, typically schizocoelous with spiral cleavage.
Comments: Various microscopic animals and others with simple body plans like the flatworms (platyhelminthes) are sometimes included in a third clade, Platyzoa, but this seems to be an artificial assemblage of mostly miniaturised spiralians. MAK120423
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