Microscopic marine worms of uncertain systematic relationship, mainly characterized by cuticular structures in the pharynx and a monociliated skin epithelium. It is the most recently described phylum of the animal kingdom. The total number of species probably exceeds 1000.
Gnathostomulids are worm-shaped, cylindrical or slightly depressed, and semitransparent (or bright red), and sometimes have the external division of head and tail. The skin is a one-layered epithelium that is completely monociliated; that is, each of the polygonal epidermal cells bears only one cilium. The sensory system usually consists of 1–2 pairs of simple and 3–4 pairs of compound bristles (frontally and laterally), and a bundle of stiff cilia (dorsally on the head). The reproductive system consists of a dorsal ovarium and in most cases two caudolateral groups of testes follicles in the same specimen. Fertilization is internal.
The distribution of gnathostomulids is worldwide, the majority of localities being known from European coasts, some from the North American east coast, and some scattered over the western Pacific.