The origin of the multicellular animals has been investigated by rate invariant analysis of 18S rRNA sequences. These analyses indicate that (i) the Metazoa is a monophyletic taxon; (ii) the Deuterostomia is a monophyletic taxon; (iii) the Annelida-Mollusca lineage is the sister group of an arthropod subgroup; and (iv) the last common ancestor of the Annelida-Mollusca lineage is most parsimoniously derived from a segmented, hemocoelic ancestor with an open circulatory system.
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James A. Lake (1990) studied this question.
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