Gnathostomulids, or jaw worms, are a small phylum of microscopic marine animals. They appear closely related to rotifers and their allies, together making up the Gnathifera. Gnathostomulids have no fossil record.
Gnathostomulids are hermaphroditic with only a single ovary.
The eggs leave the body by bursting out through the adults body wall, healing of the adult is quick and there is no larval stage, the egg hatching directly into a small but adult gnathostomulid.
Individuals go through one or more feeding but non-reproductive stages followed by a non-feeding but reproductive stage in their lives, and in some species at least it may take a year to complete a life cycle.
Gnathostomulids are apparently unselective about where or whom they inject with sperm- since it may be found in the wrong body regions and organs and sterile phase individuals are just as likely to be inseminated as sexually mature ones.
Fertilization is internal, some males use hypodermic insemination, others, like gnathostomulids, have penes that are adhesive and attach to the body wall of a female-which may drag the male around for some time.
The Lansing effect has been found in rotifers (King, 1983), that is that the offspring of old parents tend to have shorter lifespans than the offspring of younger rotifers.
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