Kingdom Animalia early animal radiation prelude: phylum choanoflagellata phylum porifera phylum placozoa eumetazoa phylum cnidaria tripoblastica phylum ctenophora phylum bilateria protostoma spiralia schizocoelia phylum sipuncula articulata phylum mollusca euarticulata phylum annelida panarthropoda phylum onychophora phylum arthropoda phylum tardigrada bryozoa phylum entoprocta phylum ectoprocta parenchymia phylum platyhelminthes phylum nemertini gnath ifera phylum rotifera phylum gnathostomulida phylum chaetognatha cycloneuralia phylum gastrotricha introverta. (Part contents)
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A 2012 study studied this question.