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Nematode

2007 Schools Wikipedia Selection. Related subjects: Organisms

                 iNematodes
         Scientific classification

   Kingdom: Animalia
   Phylum:  Nematoda
            Rudolphi, 1808

                                   Classes

   Adenophorea
      Subclass Enoplia
      Subclass Chromadoria
   Secernentea
      Subclass Rhabditia
      Subclass Spiruria
      Subclass Diplogasteria

   The nematodes or roundworms ( Phylum Nematoda from Greek νῆμα (nema):
   "thread" + ode "like") are one of the most common phyla of animals,
   with over 20,000 different described species (over 15,000 are
   parasitic). They are ubiquitous in freshwater, marine, and terrestrial
   environments, where they often outnumber other animals in both
   individual and species counts, and are found in locations as diverse as
   Antarctica and oceanic trenches. Further, there are a great many
   parasitic forms, including pathogens in most plants and animals, humans
   included. Only the Arthropoda are more diverse.

   The Nematodes were originally named Nematoidea by Rudolphi ( 1808).
   They were renamed Nematodes by Burmeister 1837 (as a family; Leuckart
   1848 and von Siebold 1848 both promoted them to the rank of order),
   then Nematoda ( Diesing 1861), though Nathan Cobb ( 1919) argued that
   they should be called Nemata or Nemates (and in English 'nemas' rather
   than 'nematodes'). After some confusion which saw the nematodes placed
   (often together with the horsehair worms, Nematomorpha) as a class or
   order in various groups such as Aschelminthes, Lankester ( 1877)
   definitively promoted them to the level of phylum.

Morphology

   Nematodes are triploblastic protostomes with a complete digestive
   system. Roundworms have no circulatory or respiratory systems so they
   use diffusion to breathe and for circulation of substances around their
   body. They are thin and are round in cross section, though they are
   actually bilaterally symmetric. Nematodes are one of the simplest
   animal groups to have a complete digestive system, with a separate
   orifice for food intake and waste excretion, a pattern followed by all
   subsequent, more complex animals. The body cavity is a pseudocoel
   (persistent blastula), which lacks the muscles of coelomate animals
   used to force food down the digestive tract. Nematodes thus depend on
   internal/external pressures and body movement to move food through
   their digestive tracts. The mouth is often surrounded by various flaps
   or projections used in feeding and sensation. The portion of the body
   past the anus or cloaca is called the "tail." The epidermis secretes a
   layered cuticle made of keratin that protects the body from drying out,
   from digestive juices, or from other harsh environments, as well as in
   some forms sporting projections such as cilia that aid in locomotion.
   Although this cuticle allows movement and shape changes via a
   hydrostatic skeletal system, it is very inelastic so does not allow the
   volume of the worm to increase. Therefore, as the worm grows, it has to
   moult and form new cuticles. The cuticles don't allow volume to
   increase so as to keep hydrostatic pressure inside the organism very
   high. For this reason, the roundworms do not possess circular muscles
   (just longitudinal ones) as they're not required. This hydrostatic
   pressure is the reason the roundworms are round.

   Most free-living nematodes are microscopic, though a few parasitic
   forms can grow to several meters in length (typically as parasites of
   very large animals such as whales). There are no circular muscles, so
   the body can only undulate from side to side. Contact with solid
   objects is necessary for locomotion; its thrashing motions vary from
   mostly to completely ineffective at swimming.

   Nematodes generally eat bacteria, fungi and protozoans, although some
   are filter feeders. Excretion is through a separate excretory pore.

   Reproduction is usually sexual. Males are usually smaller than females
   (often very much smaller) and often have a characteristically bent tail
   for holding the female for copulation. During copulation, one or more
   chitinized spicules move out of the cloaca and are inserted into
   genital pore of the female. Amoeboid sperm crawl along the spicule into
   the female worm.

   Eggs may be embryonated or unembryonated when passed by the female,
   meaning that their fertilized eggs may not yet be developed. In
   free-living roundworms, the eggs hatch into larva, which eventually
   grow into adults; in parasitic roundworms, the life cycle is often much
   more complicated.

   Nematodes have a simple nervous system, with a main nerve cord running
   along the ventral side. Sensory structures at the anterior end are
   called amphids, while sensory structures at the posterior end are
   called phasmids.

Free-living species

   In free-living species, development usually consists of four molts of
   the cuticle during growth. Different species feed on materials as
   varied as algae, fungi, small animals, fecal matter, dead organisms and
   living tissues. Free-living marine nematodes are important and abundant
   members of the meiobenthos. One roundworm of note is Caenorhabditis
   elegans, which lives in the soil and has found much use as a model
   organism. C. elegans has had its entire genome sequenced, as well as
   the developmental fate of every cell determined, and every neuron
   mapped.

Parasitic species

   Parasitic forms often have quite complicated life cycles, moving
   between several different hosts or locations in the host's body.
   Infection occurs variously by eating uncooked meat with larvae in it,
   by entrance into unprotected cuts or directly through the skin, by
   transfer via blood-sucking insects, and so forth.

   Nematodes commonly parasitic on humans include whipworms, hookworms,
   pinworms, ascarids, and filarids. The species Trichinella spiralis,
   commonly known as the trichina worm, occurs in rats, pigs, and humans,
   and is responsible for the disease trichinosis. Baylisascaris usually
   infests wild animals but can be deadly to humans as well. Haemonchus
   contortus is one of the most abundant infectious agents in sheep around
   the world, causing great economic damage to sheep farms. In contrast,
   entomopathogenic nematodes parasitize insects and are considered by
   humans to be beneficial.

   One form of nematode is entirely dependent upon the wasps which are the
   sole source of fig fertilization. They prey upon the wasps, riding them
   from the ripe fig of the wasp's birth to the fig flower of its death,
   where they kill the wasp, and their offspring await the birth of the
   next generation of wasps as the fig ripens.

   Plant parasitic nematodes include several groups causing severe crop
   losses. The most common genera are: Aphelenchoides (foliar nematodes),
   Meloidogyne (root-knot nematodes), Heterodera, Globodera (cyst
   nematodes) such as the potato root nematode, Nacobbus, Pratylenchus
   (lesion nematodes), Ditylenchus, Xiphinema, Longidorus, Trichodorus.
   Several phytoparasitic nematode species cause histological damages to
   roots, including the formation of visible galls (Meloidogyne) which are
   useful characters for their diagnostic in the field. Some nematode
   species transmit plant viruses through their feeding activity on roots.
   One of them is Xiphinema index, vector of GFLV (Grapevine Fanleaf
   Virus), an important disease of grapes.

   Other nematodes attack bark and forest trees. The most important
   representative of this group is Bursaphelenchus xylophilus, the pine
   wood nematode, present in Asia and America and recently discovered in
   Europe.

Gardening

   Soybean cyst nematode and egg
   Enlarge
   Soybean cyst nematode and egg

   Depending on the species, a nematode may be beneficial or detrimental
   to a gardener's cause.

   From a gardening perspective, there are two categories of nematode,
   predatory ones which will kill garden pests like cutworms, and pest
   nematodes like the root-knot nematode, which attack garden plants.

   Predatory nematodes can be bred by soaking a specific recipe of leaves
   and other detritus in water, in a dark, cool place, and can even be
   purchased as an organic form of pest control.

Phylogeny

   Current studies suggest that roundworms (nematodes) are related to the
   arthropods and priapulids in a newly recognized group, the Ecdysozoa
   (molting animals).

Trivia

     * Hundreds of nematode worms (C. elegans), featuring in a research
       project on mission STS-107, survived the Space Shuttle Columbia
       Disaster .

     * In the Spongebob Squarepants episode Home Sweet Pineapple, his
       house is consumed by a swarm of nematodes.

     * On the BBC2 quiz show QI, when Clive Anderson was asked, "What
       lives in the Dead Sea?", he answered, "I'm tempted to say nematode
       worms because they live everywhere."

     * In the SpongeBob SquarePants episode Best Day Ever, a swarm of
       nematodes try to eat the Krusty Krab until SpongeBob plays his nose
       as a flute. That also referes to the SpongeBob SquarePants
       beginning of an episode.

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