Roundworm

The roundworms (Phylum Nematoda) are one of the
most common phyla of animals, with over 20000
different described species. They are ubiquitous
in freshwater, marine, and terrestrial            Scientific classification

environments, where they often outnumber other   Kingdom: Animalia
animals in both individual and species counts.
Further, there are a great many parasitic forms, Phylum:  Nematoda

including pathogens in most plants and animals,            Classes
humans included. Only the Arthropoda are more    Class Adenophora
diverse.                                         ΚΚΚ Subclass Enoplia
                                                 ΚΚΚ Subclass Chromadoria
Roundworms are triploblastic protostomes. They   Class Secernentea
are shaped like stereotypical worms, long and    ΚΚΚ Subclass Rhabditia
round in cross section, though without any       ΚΚΚ Subclass Spiruria
segmentation. The body cavity is reduced to a    ΚΚΚ Subclass Diplogasteria
narrow pseudocoelom, as is typical of very small
animals. The mouth is anterior, and often surrounded by various flaps or
projections used in feeding and sensation, with the anus slightly offset
from the posterior. The epidermis secretes a layered cuticle that protects
the body from drying out, from digestive juices, or from other harsh
environments, as well as in some forms sporting projections that aid in
locomotion.

Most free-living nematodes are microscopic, though a few parasitic forms can
grow several metres in length. There are no circular muscles, so the body
can only undulate from side to side. In order to actually get anywhere, the
worm needs to be in contact with solid objects, its thrashing motions
varying from mostly to completely ineffective at swimming. Different species
feed on materials as varied as algae, fungi, small animals, fecal matter,
dead organisms and living tissues.

Reproduction is usually sexual, typically with males slightly smaller than
females and having a characteristically bent tail. In free-living species
development is usually direct, with four molts of the cuticle occurring
during growth. 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, by transfer via blood-sucking insects, and
so forth.

Important parasites on humans include whipworms, hookworms, pinworms,
ascarids, and filarids. Another roundworm of note is Caenorhabditis elegans,
which lives in the soil and has found much use as a model organism.

The common presence of a pseudocoelom is not longer considered evidence that
the pseudocoelomate phyla are all related, but a few groups are still
probably close relatives of the Nematoda. Of special note here are the
Nematomorpha, or horse-hair worms, which have larvae parasitic in arthropods
and free-living adults. The Arthropods have also been considered to be
possible relatives of these groups, the common process of ecdysis (molting)
being evidence for this.

The roundworms were originally named the Nemata by Nathan Cobb in 1919.
Later they were demoted to a class Nematoda in the Aschelminthes, and then
restored to phylum level.

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