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Jaw worms
Gnathostomula paradoxa Sylt.tif
Gnathostomula paradoxa
Scientific classification Red Pencil Icon.png
Kingdom: Animalia
Clade: Gnathifera
Ax, 1956 [1]
Orders and suborders

Gnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine animals. They inhabit sand and mud beneath shallow coastal waters and can survive in relatively anoxic environments. They were first recognised and described in 1956. [1]

In biology, a phylum is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature for algae, fungi, and plants accepts the terms as equivalent. Depending on definitions, the animal kingdom Animalia or Metazoa contains approximately 35 phyla, the plant kingdom Plantae contains about 14, and the fungus kingdom Fungi contains about 8 phyla. Current research in phylogenetics is uncovering the relationships between phyla, which are contained in larger clades, like Ecdysozoa and Embryophyta.

Animal kingdom of motile multicellular eukaryotic heterotrophic organisms

Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 millionths of a metre to 33.6 metres (110 ft) and have complex interactions with each other and their environments, forming intricate food webs. The category includes humans, but in colloquial use the term animal often refers only to non-human animals. The study of non-human animals is known as zoology.

Hypoxia refers to low oxygen conditions. Normally, 20.9% of the gas in the atmosphere is oxygen. The partial pressure of oxygen in the atmosphere is 20.9% of the total barometric pressure. In water however, oxygen levels are much lower, approximately 1%, and fluctuate locally depending on the presence of photosynthetic organisms and relative distance to the surface.



Most gnathostomulids measure 0.5 to 1 millimetre (0.020 to 0.039 in) in length. They are often slender to thread-like worms, with a generally transparent body. In many Bursovaginoidea, one of the major group of gnathostomulids, the neck region is slightly narrower than the rest of the body, giving them a distinct head. [2]

Bursovaginoidea is one of the two orders in the phylum Gnathostomulida.

Like flatworms they have a ciliated epidermis, but in contrast to flatworms, they have one cilium per cell. [3] The cilia allow the worms to glide along in the water between sand grains, although they also use muscles, allowing the body to twist or contract, for movement.

In zoology, the epidermis is an epithelium that covers the body of a eumetazoan. Eumetazoa have a cavity lined with a similar epithelium, the gastrodermis, which forms a boundary with the epidermis at the mouth.

They have no body cavity, and no circulatory or respiratory system. The nervous system is 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. [2]

Coelom The main body cavity in most animals

The coelom is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, it remains undifferentiated.

The mouth is located just behind the head, after a rostrum, on the underside of the body. It has a pair of cuticular jaws, supplied by strong muscles, and often bearing minute teeth. A "basal plate" on the lower surface that bears a comb-like structure is also present. The basal plate is used to scrape smaller organisms off of the grains of sand that make up their anoxic seabed mud habitat. [4] This bilaterally symmetrical pharynx with its complex cuticular mouth parts make them appear closely related to rotifers and their allies, together making up the Gnathifera. The ultrastructure of the jaws made of rods with electron dense core in transmission electron microscopy sections also support their close relation with Rotifera and Micrognathozoa. The mouth opens into a blind-ending tube in which digestion takes place; there is no true anus. [2] However, there is tissue connecting the intestine to the epidermis which may serve as an anal pore. [5]

Pharynx part of the throat that is behind the mouth and nasal cavity

The pharynx is the part of the throat behind the mouth and nasal cavity and above the esophagus and larynx, or the tubes going down to the stomach and the lungs. It is found in vertebrates and invertebrates, though its structure varies across species.

Rotifer phylum of animals

The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals.

Transmission electron microscopy

Transmission electron microscopy is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a grid. An image is formed from the interaction of the electrons with the sample as the beam is transmitted through the specimen. The image is then magnified and focused onto an imaging device, such as a fluorescent screen, a layer of photographic film, or a sensor such as a scintillator attached to a charge-coupled device.


Gnathostomulids are simultaneous hermaphrodites. Each individual possesses a single ovary 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. [2]

Ovary ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system

The ovary is an organ found in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilised by a sperm. There is an ovary found on the left and right sides of the body. The ovaries also secrete hormones that play a role in the menstrual cycle and fertility. The ovary progresses through many stages beginning in the prenatal period through menopause. It is also an endocrine gland because of the various hormones that it secretes.


There are approximately 100 described species [6] 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. [4]

Gnathostomulids have no known fossil record, though there are (debatable) similarities between the jaws of modern gnathostomulids and certain conodont elements. (Ochietti & Cailleux, 1969; Durden et al, 1969) [7]

They appear to be a sister clade to the Syndermata. [2] However, there is tissue connecting the intestine to the epidermis which may serve as an anal pore. [8]

Related Research Articles

Anatomy The study of the structure of organisms and their parts

Anatomy is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science which deals with the structural organization of living things. It is an old science, having its beginnings in prehistoric times. Anatomy is inherently tied to developmental biology, embryology, comparative anatomy, evolutionary biology, and phylogeny, as these are the processes by which anatomy is generated over immediate (embryology) and long (evolution) timescales. Anatomy and physiology, which study (respectively) the structure and function of organisms and their parts, make a natural pair of related disciplines, and they are often studied together. Human anatomy is one of the essential basic sciences that are applied in medicine.

Invertebrate Animals without a vertebrate column

Invertebrates are animals that neither possess nor develop a vertebral column, derived from the notochord. This includes all animals apart from the subphylum Vertebrata. Familiar examples of invertebrates include arthropods, mollusks, annelids, and cnidarians.

Sipuncula phylum of animals

The Sipuncula or Sipunculida is a group containing about 162 species of bilaterally symmetrical, unsegmented marine worms. The name Sipuncula is from the genus name Sipunculus, and comes from the Latin siphunculus meaning a "small tube". Traditionally considered a phylum, Sipuncula seems to be closely related to Myzostomida and Annelida, and may be a subgroup of Annelida, based on recent molecular work. Sipunculans vary in size but most species are under 10 cm (4 in) in length.

Gastrotrich phylum of microscopic pseudocoelomate animals

The gastrotrichs, commonly referred to as hairybacks, are a group of microscopic (0.06-3.0 mm), worm-like, pseudocoelomate animals, and are widely distributed and abundant in freshwater and marine environments. They are mostly benthic and live within the periphyton, the layer of tiny organisms and detritus that is found on the seabed and the beds of other water bodies. The majority live on and between particles of sediment or on other submerged surfaces, but a few species are terrestrial and live on land in the film of water surrounding grains of soil. Gastrotrichs are divided into two orders, the Macrodasyida which are marine, and the Chaetonotida, some of which are marine and some freshwater. Nearly eight hundred species of gastrotrich have been described.

Chaetognatha phylum of marine worms

Chaetognatha, meaning bristle-jaws, and commonly known as arrow worms, is a phylum of predatory marine worms that are a major component of plankton worldwide. About 20% of the known species are benthic, and can attach to algae and rocks. They are found in all marine waters, from surface tropical waters and shallow tide pools to the deep sea and polar regions. Most chaetognaths are transparent and are torpedo shaped, but some deep-sea species are orange. They range in size from 2 to 120 millimetres.

Onychophora Phylum of animals

Onychophora, commonly known as velvet worms or more ambiguously as peripatus, is a phylum of elongate, soft-bodied, many-legged panarthropods. In appearance they have variously been compared to worms with legs, caterpillars, and slugs. They prey upon smaller animals such as insects, which they catch by squirting an adhesive slime.

Nemertea Phylum of worms

Nemertea is a phylum of invertebrate animals also known as "ribbon worms" or "proboscis worms". Alternative names for the phylum have included Nemertini, Nemertinea and Rhynchocoela.. Most are very slim, usually only a few millimeters wide, although a few have relatively short but wide bodies. Many have patterns of yellow, orange, red and green coloration.

Bilateria all animals having a bilateral symmetry

The bilateria, bilaterians, or triploblasts, are animals with bilateral symmetry, i.e., they have a head (anterior) and a tail (posterior) as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side.

Entoprocta phylum of marine animals

Entoprocta whose name means "anus inside", or Kamptozoa, is a phylum of mostly sessile aquatic animals, ranging from 0.1 to 7 millimetres long. Mature individuals are goblet-shaped, on relatively long stalks. They have a "crown" of solid tentacles whose cilia generate water currents that draw food particles towards the mouth, and both the mouth and anus lie inside the "crown". The superficially similar Bryozoa (Ectoprocta) have the anus outside a "crown" of hollow tentacles. Most families of entoprocts are colonial, and all but 2 of the 150 species are marine. A few solitary species can move slowly.

Turbellaria class of worms

The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes (flatworms), and include all the sub-groups that are not exclusively parasitic. There are about 4,500 species, which range from 1 mm (0.039 in) to large freshwater forms more than 500 mm (20 in) long or terrestrial species like Bipalium kewense which can reach 600 mm (24 in) in length. All the larger forms are flat with ribbon-like or leaf-like shapes, since their lack of respiratory and circulatory systems means that they have to rely on diffusion for internal transport of metabolites. However, many of the smaller forms are round in cross section. Most are predators, and all live in water or in moist terrestrial environments. Most forms reproduce sexually and with few exceptions all are simultaneous hermaphrodites.

<i>Limnognathia</i> monotypic taxon

Limnognathia maerski is a microscopic platyzoan freshwater animal, discovered living in cold springs on Disko Island, Greenland in 1994, that has variously been assigned as a class or subphylum in the phylum Gnathifera or as a phylum in a Gnathifera superphylum, named Micrognathozoa. It is related to the rotifers and gnathostomulids, grouped together as the Gnathifera. With an average length of one-tenth of a millimetre, it is one of the smallest animals known.

Platyzoa superphylum of animals

The paraphyletic "Platyzoa" are a group of protostome unsegmented animals proposed by Thomas Cavalier-Smith in 1998. Cavalier-Smith included in Platyzoa the phylum Platyhelminthes, and a new phylum, the Acanthognatha, into which he gathered several previously described phyla of microscopic animals. More recently it has been described as paraphyletic, containing the Rouphozoa and the Gnathifera.

Acorn worm class of hemichordates

The acorn worms or Enteropneusta are a hemichordate class of invertebrates consisting of one order of the same name. Their closest relatives are the echinoderms. There are 111 known species of acorn worm in the world, the main species for research being Saccoglossus kowalevskii. Two families - Harrimaniidae and Ptychoderidae - separated at least 370 million years ago.

Phoronid phylum of marine animals, horseshoe worms

Phoronids are a small phylum of marine animals that filter-feed with a lophophore, and build upright tubes of chitin to support and protect their soft bodies. They live in most of the oceans and seas including the Arctic Ocean but excluding the Antarctic Ocean, and between the intertidal zone and about 400 meters down. Most adult phoronids are 2 cm long and about 1.5 mm wide, although the largest are 50 cm long.

Nematode phylum of animals with tubular digestive systems with openings at both ends

The nematodes or roundworms constitute the phylum Nematoda. They are a diverse animal phylum inhabiting a broad range of environments. Taxonomically, they are classified along with insects and other moulting animals in the clade Ecdysozoa, and unlike flatworms, have tubular digestive systems with openings at both ends.

Sucker (zoology) specialised attachment organ of an animal

A sucker in zoology refers to specialised attachment organ of an animal. It acts as an adhesion device in parasitic worms, several flatworms, cephalopods, certain fishes, amphibians, and bats. It is a muscular structure for suction on a host or substrate. In parasitic annelids, flatworms and roundworms, suckers are the organs of attachment to the host tissues. In tapeworms and flukes, they are a parasitic adaptation for attachment on the internal tissues of the host, such as intestines and blood vessels. In roundworms and flatworms they serve as attachment between individuals particularly during mating. In annelids, a sucker can be both a functional mouth and a locomotory organ. The structure and number of suckers are often used as basic taxonomic diagnosis between different species, since they are unique in each species. In tapeworms there are two distinct classes of suckers, namely "bothridia" for true suckers, and "bothria" for false suckers. In digeneal flukes there are usually an oral sucker at the mouth and a ventral sucker posterior to the mouth. Roundworms have their sucker just in front of the anus; hence it is often called a pre-anal sucker.

Annelid Phylum of segmented worms

The annelidas, also known as the ringed worms or segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecologies – some in marine environments as distinct as tidal zones and hydrothermal vents, others in fresh water, and yet others in moist terrestrial environments.

<i>Tubulanus polymorphus</i> species of worm

Tubulanus polymorphus is a species of ribbon worm in the phylum Nemertea. It is found in the northern Atlantic Ocean and the northern Pacific Ocean. It occurs on the lower shore down to about 50 m (160 ft), on sand or gravel, under stones and among seaweed.


  1. 1 2 Ax, P. (1956). "Die Gnathostomulida, eine rätselhafte Wurmgruppe aus dem Meeressand". Abhandl. Akad. Wiss. U. Lit. Mainz, math. - naturwiss. 8: 1–32.
  2. 1 2 3 4 5 Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 311–312. ISBN   0-03-056747-5.
  3. Ruppert, Edward E., Fox, Richard S., Barnes, Robert D. (2004) Invertebrate Zoology (7th edition). Brooks/Cole-Thomson Learning, Belmont, US
  4. 1 2 Barnes, R.F.K. et al. (2001). The Invertebrates: A Synthesis. Oxford: Blackwell Science.
  5. Knauss, Elizabeth (December 1979). "Indication of an Anal Pore in Gnathostomulida". Zoologica Scripta. 8 (1–4): 181–6. doi:10.1111/j.1463-6409.1979.tb00630.x via Wiley Online Library.
  6. Zhang, Z.-Q. (2011). "Animal biodiversity: An introduction to higher-level classification and taxonomic richness" (PDF). Zootaxa. 3148: 7–12.
  7. Cited in chapter 2 , p192 of Andreas Schmidt-Rhaesa'a 2015 Gastrotricha and Gnathifera, VOl 3 of Gast---, Cycloneuralia and Gnath---.
  8. Golombek, A.; Tobergte, S.; Struck, T.H. (May 2015). "Elucidating the phylogenetic position of Gnathostomulida and first mitochondrial genomes of Gnathostomulida, Gastrotricha and Polycladida (Platyhelminthes)". Mol Phylogenet Evol. 86: 49–63. doi:10.1016/j.ympev.2015.02.013. PMID   25796325.