Gnathostomulida

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Gnathostomulid, group of microscopic marine worms that live between grains of sand in shallow ocean water. These invertebrates are able to live for long periods without oxygen (see Anaerobe) and are common in stagnant bottom muck and black sand. More than 80 species of gnathostomulids have been described.

Gnathostomulids resemble flatworms and were once classified in the same phylum. Like flatworms, gnathostomulids are bilaterally symmetrical (both sides of the body are identical along the midline) and have waving body cilia (tiny hairlike projections); they lack an anus and a coelom (internal body cavity) and are hermaphroditic (individuals have both male and female sex organs). Unlike flatworms, however, each epithelial (skin) cell in gnathostomulids has only one hairlike cilium. In addition there are significant differences in sperm structure between gnathostomulids and flatworms.

Gnathostomulids are generally 0.5 to 1.0 mm (0.02 to 0.04 in) in length with a slight constriction of the body separating the head from the trunk. They move their long transparent bodies with the propelling force of their rotating cilia. Unlike flatworms, they have the ability to reverse the direction of the ciliary beat. The contractions of three or four pairs of longitudinal muscle fibers also contribute to movement. Gnathostomulids have a mouth located on the underside of the body behind the head. They graze on bacteria and fungi using a pair of toothed jaws that grasp and push food into the intestinal sac.

Even though gnathostomulids possess both male and female sex organs, they cannot fertilize themselves. Eggs are fertilized when one gnathostomulid injects a packet of sperm into another individual’s body using a penislike organ that is equipped with a piercing tip in some species. A single large egg is released by breaking through the body wall and develops without larval stages into an adult. The parent worm regenerates rapidly after releasing the egg. Some gnathostomulids appear to alternate between a nonsexual feeding phase and a sexual phase during which they do not feed.

Scientific classification: Gnathostomulids make up the phylum Gnathostomulida, and are members of the superphylum Aschelminthes.

Types of Invertebrates

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Kinorhyncha or Echinodera -> Tiny worms with spiny bodies. An outer protective cuticle is segmented and articulated. Found in the muddy bottoms of coastal waters, they feed on microorganisms and organic particles by means of a sucking pharynx. Reproduction is sexual. Examples: Echinoderes, Condyloderes

Nematoda -> Commonly known as roundworms, these animals are one of the most diverse and geographically widespread invertebrate phyla. Free-living roundworms inhabit freshwater and marine habitats, as well as soil. Parasitic roundworms prey on both plants and animals, causing widespread agricultural damage and disease. Roundworms have long, cylindrical bodies with a mouth surrounded by lips and sensory papillae or bristles. Fluid in the body cavity distributes nutrients and oxygen—roundworms do not have special respiratory or circulatory systems. Roundworms prey on other invertebrates as well as diatoms, algae, and fungi. They reproduce sexually and larvae undergo at least four molts before reaching their adult size and shape. – Examples: Ascarids, vinegar eels, cyst nematodes, heartworms, hookworms

Nematomorpha -> Long, threadlike worms found in soil or freshwater, commonly known as horsehair worms. They have no distinct head. Larvae are parasitic on terrestrial arthropods, usually insects. Adults do not feed but depend entirely on nutrients obtained during the parasitic larval stage. Reproduction is sexual. – Examples: Nectoneme, gordian worms

Acanthocephala -> Worms characterized by the presence of retractable spiny hooks that attach to the intestinal walls of aquatic and terrestrial vertebrates. Lacking a digestive tract, these worms absorb nutrients through their body wall. Reproduction is sexual and the life cycle includes larval forms that are parasites of crustaceans or insects. – Examples: Spiny-headed worms

Gnathostomulida -> Elongated, ciliated worms that live in marine sand. These animals have a mouth structure with a combed plate and toothed jaws. Bacteria and fungi are scraped into the mouth by the comb and passed into the gut by snapping movements of the jaws. Hermaphroditic, these animals reproduce sexually. – Examples: Jaw worms

Mollusca -> Diverse animals found in water and on land. Most mollusks have a hard shell that protects a soft body, although in some mollusks the hard shell is missing or hardly visible. A feeding organ called a radula contains rows of teeth used to scrape food into the mouth. Enzymes in salivary glands partially digest food before it reaches the intestines. Reproduction is sexual and some mollusks have a larval form. – Examples: Chitons, oysters, snails, clams, squid

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