Gnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine animals. They inhabit sand and mud beneath shallow coastal waters and can survive in relatively anoxic environments. They were first recognised and described in 1956.
Most gnathostomulids measure 0.5 to 1 millimetre (0.020 to 0.039 in) in length. They are slender to thread-like worms, with a transparent body. The neck region is slightly narrower than the rest of the body, giving them a distinct head.
Like flatworms they have a ciliated epidermis, but are unique in having but one cilium per cell. The cilia allow the worms to glide along in the water between sand grains, although they also have muscles that allow the body to twist or contract.
They have no body cavity, and no circulatory or respiratory system. The nervous system is very simple, and restricted to the outer layers of the body wall. The only sense organs are modified cilia, which are especially common in the head region.
The mouth is located just behind the head, on the underside of the body. It has a pair of muscular jaws supplied with minute teeth, and a plate on the lower surface that bears a comb-like structure 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. The mouth opens into a blind-ending tube in which digestion takes places; there is no anus.
Gnathostomulids are simultaneous hermaphrodites. Each individual possesses a single ovary and one or two testes. After fertilization, the single egg ruptures through the body wall and adheres to nearby sand particles; the parent is able to rapidly heal the resulting wound. The egg hatches into a miniature version of the adult, without a larval stage.
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.
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