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Early investigators suggested that the acoelomate platyhelminthes (flatworms) are basal to the bilaterian metazoans, but more recent phylogenetic analyses place them as basal members of the protostome spiralians (20, 21).
It has also been reported for crustaceans (Bell and Smith, 1993) and ascidians (Bell and Smith, 1994) but not, as yet, for acoelomate animals.
Because these acoelomate, planktonic animals lack both internal and external support systems, they are highly susceptible to injury.