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Protostomes evolved into over a million species alive today, compared to ca. 73,000 deuterostome species. [6] Protostomes are divided into the Ecdysozoa (e.g. arthropods, nematodes) and the Spiralia (e.g. molluscs, annelids, platyhelminths, and rotifers). A modern consensus phylogenetic tree for the protostomes is shown below.
The Spiralia are a morphologically diverse clade of protostome animals, including within their number the molluscs, annelids, platyhelminths and other taxa. [4] The term Spiralia is applied to those phyla that exhibit canonical spiral cleavage, a pattern of early development found in most members of the Lophotrochozoa.
Chaetognatha appears close to the base of the protostome tree in most studies of their molecular phylogeny. [35] This may explain their deuterostome embryonic characters. If chaetognaths branched off from the protostomes before they evolved their distinctive protostome embryonic characters, they might have retained deuterostome characters ...
Gnathifera is a member of Spiralia. It is the sister taxon of a clade comprising all other spiralians. [1] [19] An alternative phylogeny place Gnathifera into a main spiralian clade Platyzoa s.l. as sister clade to Mesozoa and Platyhelminthes. [21]
This clade, that is, a group consisting of a common ancestor and all its descendants, was formally named by Aguinaldo et al. in 1997, based mainly on phylogenetic trees constructed using 18S ribosomal RNA genes. [10] A large study in 2008 by Dunn et al. strongly supported the monophyly of Ecdysozoa. [11]
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Lophotrochozoa (/ l ə ˌ f ɒ t r oʊ k oʊ ˈ z oʊ ə /, "crest/wheel animals") is a clade of protostome animals within the Spiralia. The taxon was established as a monophyletic group based on molecular evidence. [2] [3] The clade includes animals like annelids, molluscs, bryozoans, and brachiopods. [4]
In species with incomplete septa or none, the blood circulates through the main body cavity without any kind of pump, and there is a wide range of locomotory techniques – some burrowing species turn their pharynges inside out to drag themselves through the sediment.