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Ruppert, E.E., Fox, R.S., and Barnes, R.D. (2004). Invertebrate Zoology (7 ed.). Brooks / Cole. p. 520. ISBN 0030259827.
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: CS1 maint: multiple names: authors list (link) who cite Ax, P. (2000). Multicellular Animals: The Phylogenetic System of the Metazoa. Vol. 2. Berlin: Springer. p. 396.
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