Gnathostomulids: Gnathostomulida

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GNATHOSTOMULIDS: Gnathostomulida



Gnathostomulids (NATH-oh-STOH-muh-lids) are tiny sea worms. They are thread shaped and 0.01 to 0.1 inch (0.3 to 3.5 millimeters) long. Most are colorless or transparent, but some are bright red. The front end of some gnathostomulids is pointed, but that of others is rounded. The rear end is rounded or forms a tail. Each cell of the body covering has a single, long hairlike fiber used for movement. Some of these fibers also may have sensing functions.

The nervous system is at the base of the body covering and consists of a brain and a structure from which paired nerves originate. The muscles are simple and weak, except for a complex feeding tube. The mouth is on the bottom of the worm near the front, and there is no anus (AY-nuhs). In most species the complex, muscular, feeding tube contains hard mouthparts consisting of a plate in the lower lip and paired jaws. In most species, the inner, front parts of the jaw have groups or rows of teeth.


Gnathostomulids live all over the world but mainly in the northern part of the Atlantic Ocean and the southern part of the Pacific Ocean.


Gnathostomulids live on sheltered beaches, near sea grasses and mangroves, and between coral reefs.


Gnathostomulids graze on bacteria and threads of fungus attached to sand grains.


Gnathostomulids glide between sand grains. They contract when disturbed. Some species spin a cocoon of mucus. Gnathostomulids make both eggs and sperm. Sperm is transferred by mating and is stored either between the digestive tract and outer tissue layer or in a storage pouch. Only one large egg matures at a time. The egg joins with sperm and then is laid by bursting through the worm's back. The eggs hatch directly into young animals, which grow to adults.


Gnathostomulids have no known importance to people.


Gnathostomulids are not considered threatened or endangered.


Physical characteristics: Red haplognathia are 0.1 inch (3.5 millimeters) long and 0.006 in (140 micrometers) in diameter. They are one of the largest gnathostomulids. Most of these worms are brick red, reddish brown, or pink. The head is pointed, and the rear is rounded. The jaws are solid and have large winglike structures and many sharp feelers. The bottom plate of the jaw is shieldlike and has thorns.

Geographic range: Red haplognathia live on the coasts of Australia, Fiji, Hawaii, eastern North America, western Europe, and countries surrounding the Mediterranean Sea.

Habitat: Red haplognathia live in waste-rich sand in shallow water near the shore.

Diet: Red haplognathia graze on fungus threads and bacteria among sand grains.

Behavior and reproduction: Red haplognathia coil up by muscular action, then uncoil using their hairlike fibers, often from both ends at the same time, the head pulling forward and the rear pulling backward. Red haplognathia lay a single egg, which bursts through the worm's back. The egg then sticks to a sand grain until the young worm emerges.

Red haplognathia and people: Red haplognathia have no known importance to people.

Conservation status: Red haplognathia are not considered threatened or endangered. ∎



Valentine, James W. On the Origin of Phyla. Chicago: University of Chicago Press, 2004.

Web sites:

Tyler, Seth. "Platyhelminthes and Acoelomorpha: Phyla of Controversy." University of Maine. (accessed on February 3, 2005).