Gnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine animals. Most measure between 0.5 and 1 mm long. Like flatworms they have a ciliated epidermis, but are unique in having but one cilium per cell. They have no body cavity, and no circulatory or respiratory system. Each gnathostomulid is simultaneously hermaphrodite, possessing an ovary and a testis. They are characterized by a specialized, muscular jaw, which they use to scrape smaller organisms off of the grains of sand that make up their anoxic seabed mud habitat. This bilaterally symmetrical pharynx with its complex cuticular mouth parts make them appear closely related to rotifers and their allies, together making up the Gnathifera.
There are approximately 100 described species and certainly many more as yet undescribed. The known species are grouped in two orders. The filospermoids are very long and are characterized by an elongate rostrum. The bursovaginoids have paired sensory organs and are characterized by the presence of a penis and a sperm-storage organ called a bursa.
Gnathostomulids have no fossil record.