Everything about The Nematode totally explained
The
nematodes or
roundworms (
Phylum Nematoda from
Greek νῆμα (nema): "thread" + -ώδη -ode "like") are one of the most common phyla of
animals, with over 80,000 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, animals, and also in humans.
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
Roundworms are unsegmented,
bilaterally symmetric and
triploblastic protostomes with a complete
digestive system. Roundworms have no circulatory or respiratory systems so they use
diffusion to breathe. Although they lack a circulatory system, nutrients are transported throughout the body via fluid in the pseudocoelom. They are thin and are round in cross section. 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
pseudocoelom (persistent
blastula), which lacks the muscles of coelomate animals that protects the body from drying out, from digestive juices, or from other harsh environments. Although this cuticle allows movement and shape changes via a
hydrostatic skeletal system, it's very inelastic so doesn't allow the volume of the worm to increase. Therefore, as the worm grows, it has to
molt 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 don't possess circular muscles (just longitudinal ones) as they're not required. This hydrostatic pressure is the reason the roundworms are round.
Nematodes have a simple nervous system, with a main
ventral nerve cord and a smaller
dorsal nerve cord. Sensory structures at the anterior end are called
amphids, while sensory structures at the posterior end are called
phasmids.
Most free-living nematodes are microscopic, though a few parasitic forms can grow to over a meter 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. Nematodes also contract bacterial infections within excretion pores.
Reproduction
Reproduction is usually sexual. Males are usually smaller than females (often 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. 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 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.
Some nematodes, specifically
Heterorhabditis spp., undergo a process called
endotokia matricida; intrauterine birth causing maternal death. The hermaphroditic nematode keeps its self-fertilized eggs inside its uterus until they hatch. The juvenile nematodes will then ingest the parent nematode. This process is significantly promoted in environments with a low or reducing food supply.
Abundance
The nematode species are very difficult to distinguish. Of the
pseudocoelomates, the nematodes are the most common. Nematodes have successfully adapted to nearly every
niche from marine to fresh water, from the polar regions to the tropics, as well as the highest to the lowest of elevations. They represent, for example, 90% of all life on the seafloor of the Earth.
(External Link
) Though 20,000 species have been classified it's estimated that this number might be upwards of 500,000 if all were known.. In certain fertile areas the
topsoil is estimated to contain in the
billions of nematodes per
acre. In the 1914 edition of the Yearbook of the
United States Department of Agriculture N.A.Cobb wrote on the abundance of nematodes
That the roundworms have a large number of peculiar
apomorphies and in many cases a parasitic lifestyle confounds analyses; the
DNA sequence data hitherto analyzed is equivocal on ecdysozoan
monophyly. Genetic analyses of roundworms suggest that - as is also indicated by their unique morphological features - the group has been under intense
selective pressure during its early radiation, resulting apparently in accelerated rates of both morphological and molecular evolution. Until a strong
phylogenetic tree based on combined evidence is produced, most agree that the Nematoda should simply be referred to as part of the
Metazoa.
Robustness
Further Information
Get more info on 'Nematode'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://nematode.totallyexplained.com">Nematode Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
