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.