Relationships
The phylogenetic placement of this phylum in the past has proved troublesome. Originally classified as annelids, despite the complete lack of segmentation, bristles and other annelid characters, the phylum Sipuncula was later allied with the Mollusca, mostly on the basis of developmental and larval characters. Currently these two phyla have been included in a larger group, the Lophotrochozoa, that also includes the annelids, the ribbon worms and several other phyla. Phylogenetic analyses based on 79 ribosomal proteins indicated a position of Sipuncula within Annelida. Subsequent analysis of the mitochondrion's DNA has confirmed their close relationship to the Myzostomida and Annelida (including echiurans and pogonophorans).
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