Gnathostomulids are hermaphrodites, either simultaneously functioning as male and female or as male first. One egg matures at a time and fertilization is apparently internal; in at least some species, sperm are apparently injected by an individual beneath the skin of its mate. Fertilized eggs are believed to be deposited singly. Development is direct, with no larval stage.
(Brusca and Brusca 2003; Sørensen and Sterrer 2009 and references therein; Margulis and Chapman 2010)
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