Gnathostomulid
Encyclopedia
Gnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

s. They inhabit sand and mud beneath shallow coastal waters and can survive in relatively anoxic
Hypoxia (environmental)
Hypoxia, or oxygen depletion, is a phenomenon that occurs in aquatic environments as dissolved oxygen becomes reduced in concentration to a point where it becomes detrimental to aquatic organisms living in the system...

 environments. They were first recognised and described in 1956.

Anatomy

Most gnathostomulids measure 0.5 to 1 mm (0.0196850393700787 to 0.0393700787401575 ) in length. They are slender to thread-like worms, with a transparent body. The neck region is slightly narrower than the rest of the body, giving them a distinct head.

Like flatworms they have a ciliated epidermis
Epidermis (zoology)
The Epidermis is an epithelium that covers the body of an eumetazoan . Eumetazoa have a cavity lined with a similar epithelium, the gastrodermis, which forms a boundary with the epidermis at the mouth.Sponges have no epithelium, and therefore no epidermis or gastrodermis...

, but are unique in having but one cilium per cell. The cilia allow the worms to glide along in the water between sand grains, although they also have muscles that allow the body to twist or contract.

They have no body cavity
Coelom
The coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...

, and no circulatory or respiratory system. The nervous system is very simple, and restricted to the outer layers of the body wall. The only sense organs are modified cilia, which are especially common in the head region.

The mouth is located just behind the head, on the underside of the body. It has a pair of muscular jaws supplied with minute teeth, and a plate on the lower surface that bears a comb-like structure which they use to scrape smaller organisms off of the grains of sand that make up their anoxic seabed mud habitat. This bilaterally symmetrical pharynx
Pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...

 with its complex cuticular mouth parts make them appear closely related to rotifer
Rotifer
The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...

s and their allies, together making up the Gnathifera
Gnathifera (phylum)
Gnathifera is an assemblage of phyla of metazoans."Acanthognatha" is a similar grouping, including rotifers, acanthocephalans, gastrotrichs, and gnathostomulids....

. The mouth opens into a blind-ending tube in which digestion takes place; there is no anus
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...

.

Reproduction

Gnathostomulids are simultaneous hermaphrodites. Each individual possesses a single ovary
Ovary
The ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in anatomically female individuals are analogous to testes in anatomically male individuals, in that they are both gonads and endocrine glands.-Human anatomy:Ovaries...

 and one or two testes. After fertilization, the single egg ruptures through the body wall and adheres to nearby sand particles; the parent is able to rapidly heal the resulting wound. The egg hatches into a miniature version of the adult, without a larval stage.

Taxonomy

There are approximately 100 described species and certainly many more as yet undescribed. The known species are grouped in two orders. The filospermoids are very long and are characterized by an elongate rostrum. The bursovaginoids have paired sensory organs and are characterized by the presence of a penis and a sperm-storage organ called a bursa.

Gnathostomulids have no fossil record.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK