Not all authorities place gnathostomulids in a separate phylum. A classification in Invertebrate Zoology considers them a class in the phylum Aschelminthes, and other zoologists undoubtedly have different ideas. Some evolutionary zoologists seem to put them in a separate phylum because they cannot decide where they fit in their evolutionary scheme.
It seems that the observable characteristics of the gnathostomulids are sufficiently different from those of similar animals to give them a phylum of their own. While they might be conveniently placed with Aschelminthes or some other group, the phylum Gnathostomulida is well established in literature, so I strongly advocate its retention. Taxonomy ought to be a convenient, easily-learned system in which similar species are grouped together in a more or less logical fashion. Constantly shifting animals from one classification to another hinders the learning process.
External Form
According to Invertebrate Zoology, “Gnathostomulids are small and worm-like, generally less than 1 mm long and about 50 μm in diameter.” Other sources that I consulted are in essential agreement with these figures.
Gnathostomulids possess bilateral symmetry. They are covered with a ciliated epidermis. Each epidermal cell possesses a single cilium. Their mouth is not at the front tip of their body, but a little ways back on the ventral (bottom) side. The front part of the animal also has some long stiff cilia that function as sensory receptors.
The posterior (back) part of the body is tapered. Near the end of the animal, there is a male genital opening, but there is no anus, according to Invertebrate Zoology.
Inside the Body
The entire digestive system of gnathostomulids is located in the anterior (front) part of the animal’s body. It consists of the mouth, a muscular pharynx with a basal plate, and a gut with a layer of large cells. Especially noteworthy are large lateral jaws, which give the phylum its name. Gnathostomulida comes from the Greek words gnáthos, which means “jaw,” and stoma, which means “mouth.”
The back part of its body contains the reproductive apparatus, which is more complex in some species than in others. In the more complex forms, there is an ovary behind the gut, followed by a structure called a bursa. Further back are paired testes. The less complex forms do not have a bursa, a vagina, or a penis, according to General Zoology.
Beneath the epidermis, there is a thin layer of circular muscles. Beneath the circular muscles are paired groups of longitudinal muscle fibers, according to General Zoology.
There is no coelom or even a pseudocoel between these muscles and the internal organs of digestion and reproduction. This area contains parenchyma, which is a somewhat disorganized collection of cells. There is no circulatory system.
According to General Zoology, “the nervous system is simple and lies beneath the epidermis. Nervous tissue is concentrated near the anterior end.”
According to Invertebrate Zoology, gnathostomulids have special excretory cells called cyrtocytes in paired groups near the testes. Cyrtocytes also occur in the parenchyma.
Life Cycle
Gnathostomulids are hermaphroditic. However, “male and females sometimes occur, sometimes after temporary resorption of the opposite sex organ,” according to Invertebrate Zoology.
After internal fertilization, the fertilized egg leaves the parent through the body wall. By the time its first free swimming stage develops, its form is similar to that of adults, except that they do not have all the structures that the adult enjoys. For example, young gnathostomulids lack jaws.
Lifestyle
The range of gnathostomulids is extensive. You can find them in the ocean, not only in warm tropical waters, but also in waters of the temperate and frigid zones.
Gnathostomulids seem to be buried alive. They live embedded in sand and silt near shore or under moderately deep waters. Fortunately, they do not need much oxygen.
They typically cling to a particle of sand. According to Invertebrate Zoology, they prefer particles that have a diameter larger than one tenth of a millimeter and smaller than one fourth of a millimeter, probably because they have difficulty living among smaller particles, while nematodes and other predators can easily catch them among larger particles.
Their food seems to be fungi and bacteria, according to General Zoology.
References:
Invertebrate Zoology by Joseph Engemann and Robert Hegner
General Zoology by Tracy Storer, Robert Stebbins, Robert Usinger, and James Nybakken