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The myzostomes are animals with five pairs of parapodia, living as commensals or (endo)parasites mostly on crinoid and ophiuroid echinoderms. They are generally considered aberrant annelids, possibly phyllodocidan polychaetes. A phylogenetic analysis of 18S and 28S ribosomal DNA sequence data of Myzostoma glabrum , together with 60 morphological, developmental, ultrastructural, and life-history characters, is presented to show that myzostomes are a sister group of the Cycliophora, closely related to the rotifer-acanthocephalan clade (= Syndermata). Myzostomes and syndermates share predominantly the highly derived spermatozoa with anteriorly directed flagella (cycliophoran sperm is insufficiently known). The myzostome-cycliophoran-syndermate clade, accommodated within the Platyzoa (including Platyhelminthes s. str. , Gastrotricha, Gnathostomulida, Syndermata, Cycliophora, and Myzostomida), is strongly supported by most analyses, regardless of alignment parameters, character combinations and weighting, species sampling, and tree-building methods. The new name Prosomastigozoa (“forward-flagellar animals”) is proposed for the group including three phyla (Cycliophora, Myzostomida, and Syndermata). |
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