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Nematodes
The nematodes or roundworms (Phylum
Nematoda) are the most diverse phylum
of pseudocoelomates , and one of the
most diverse of all animals.
They are very difficult to distinguish
(28,000 have been described and
undescribed roundworms might be more
than 500,000).
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HABITATThey are ubiquitous in freshwater, marine and terrestrial environments.
Found in locations as diverse in
Antarctica and oceanic trenches (90% ofall life on the seafloor of the Earth.)
Some are found in most plants and
animals (pathogens)Others undergo an ametabolic state to
respond to environmental conditions
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Systematics and
Phylogeny
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Phylum Nematoda Basal OrderMonhysterida
Basal subclass/Class Enoplia
Basal subclass/Class Dorylaimia
Class Secernentea
Subclass Diplogasteria
Subclass Rhabditia
Subclass Spiruria
Subclass Tylenchia
Chromadoria assemblage
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Physical Structures
Slender worm-like animals, typically less than 2.5mm.(smallest are microscopic, while others grow more than
5cm)
usually ornamented with rings, ridges, warts,
bristles or other distinctive structures
the mouth has 3 or 6 lips, the head is radially
symmetrical in many cases and
the rest of body is bilaterally symmetrical.
Epidermis is covered by a thick cuticle with muscles
underneath that covers the coelem
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Digestive System
The mouth often includes a hollow and sharp stylet that can beused to suck liquids from plants and animals
It has no stomach because its pharynx already
connects directly to its intestine and its wastes areexpelled through its anus. Its intestines have
valves which help control the movement of the
food in the body
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Excretory System
Nitrogenous waste is excreted in the form of ammonia through thebody walls.
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Nervous system
Four nerves run the length of the body on thedorsal, ventral, and lateral surfaces. Each lies
within a cord of connective tissue lying
beneath the cuticle and between the muscle
cells.
The ventral nerve is the largest, and has a
double structure forward of the excretory pore.
The dorsal nerve is responsible for motorcontrol, while the lateral nerves are sensory,
and the ventral combines both functions.
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At the anterior end of the animal, the nerves
branch from a dense circular nerve ring
surrounding the pharynx, and serving as thebrain.
Smaller nerves run forward from the ring to
supply the sensory organs of the head.thebody is covered with numerous number of
bristles and papillae that together provides a
sense of touch.
Behind the sensory bristles on the head lie
two small pits, oramphids.
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Amphids are well supplied with nerve
cells and are probably chemoreception
organs.
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Reproduction
Most species are dioecious, with
separate male and female individuals.
Both sexes possess one or two tubular
gonads.
In males, sperm is produced at the end
of the gonad and migrates along its
length as it mature
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The testes each open into a relatively wide
sperm duct and then into a glandular andmuscular ejaculatory duct associated with the
cloaca. In females, the ovaries each open into
an oviduct and then a glandular uterus. Theuteri both open into a common vagina, usually
located in the middle of the ventral surface.
Reproduction is usually sexual. Males are
usually smaller than females (often much
smaller)
http://en.wikipedia.org/wiki/Cloacahttp://en.wikipedia.org/wiki/Cloaca -
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Nematode sperm is thought to be the only
eukaryotic cell without the globular protein G-actin.
Eggs may be embryonated or unembryonated
when passed by the female, meaning that theirfertilized eggs may not yet be developed. A few
species are known to be ovoviviparous. The eggs
are protected by an outer shell, secreted by theuterus. In free-living roundworms, the eggs hatch
into larva, which appear essentially identical to the
adults, except for an under-developed
reproductive system; in parasitic roundworms, thelife cycle is often much more complicated.
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Some nematodes, such as Heterorhabditis spp.,
undergo a process called endotokia matricida:
intrauterine birth causing maternal death.Somenematodes are hermaphroditic, and keep their
self-fertilized eggs inside the uterus until they
hatch. The juvenile nematodes will then ingest theparent nematode. This process is significantly
promoted in environments with a low or reducing
food supply.
http://en.wikipedia.org/wiki/Heterorhabditishttp://en.wikipedia.org/w/index.php?title=Endotokia_matricida&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Endotokia_matricida&action=edit&redlink=1http://en.wikipedia.org/wiki/Heterorhabditis -
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Free-living species
Usually consists of four molts of cuticle duringgrowth
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
They play an important role in the decompositionprocess, aid in recycling of nutrients in marine
environments and are sensitive to changes in the
environment caused by pollution.
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Parasitic Species
Nematodes commonly parasitic on humans
include ascarids (Ascaris), filarids, hookworms,pinworms (Enterobius) and whipworms (Trichuris
trichiura).
The species Trichinella spiralis, commonlyknown 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.
http://en.wikipedia.org/wiki/Trichinella_spiralishttp://en.wikipedia.org/wiki/Baylisascarishttp://en.wikipedia.org/wiki/Baylisascarishttp://en.wikipedia.org/wiki/Trichinella_spiralis -
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One form of nematode is entirely dependent upon
fig 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 figflower 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.
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The most common genera areAphelenchoides (
foliar nematodes), Ditylenchus, Globodera (potato
cyst nematodes), etc.
Several phytoparasitic nematode species causehistological damages to roots, including the
formation of visible galls (e.g. by root-knot
nematodes), which are useful characters for theirdiagnostic in the field.
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.
http://en.wikipedia.org/wiki/Aphelenchoideshttp://en.wikipedia.org/w/index.php?title=Foliar_nematode&action=edit&redlink=1http://en.wikipedia.org/wiki/Ditylenchushttp://en.wikipedia.org/wiki/Globoderahttp://en.wikipedia.org/wiki/Bursaphelenchus_xylophilushttp://en.wikipedia.org/wiki/Bursaphelenchus_xylophilushttp://en.wikipedia.org/wiki/Globoderahttp://en.wikipedia.org/wiki/Ditylenchushttp://en.wikipedia.org/w/index.php?title=Foliar_nematode&action=edit&redlink=1http://en.wikipedia.org/wiki/Aphelenchoides