Gnathostomulida
.
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.
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.
Comments