The gut is a simple tube or sac. The head has a number of sensory projections and pits of cilia on the epidermis, with nervous tissue concentrated in cephalic and buccal ganglia. The complex oral apparatus lies on the ventral surface of the head. Encrusted bacteria and fungi are scraped up by the basal plate, and chopped by the jaws before passing into the gut. Gnathostomulids are interstitial animals, whose usual habitats are fine, anoxic marine sediments.
All movement is achieved through ciliary gliding, swimming and contortion of the body.
The majority of Gnathostomulids are hermaphrodite, although monoecious forms are also known. Fertilisation occurs internally with the production of a single large egg. The egg later escapes from the body by breaking directly through the body wall. Cleavage of the egg is spiral, and the development to adulthood is direct. Classification of the Gnathostomulids is an area of conjecture, with a number of features being shared with the turbellarian flatworms, with whom they were formerly included.