Nematodes are a diverse phylum of roundworms found in nearly every environment. They range in size and can be free-living or parasitic. Their bodies are elongated and cylindrical with an outer cuticle layer and internal longitudinal muscles. They lack circulatory and respiratory systems. The phylum is divided into three main classes - Chromadorea, Enoplea, and Secernentea - which are differentiated based on sensory structures and anatomy. Chromadorea inhabit marine sediments and have complex pharynx. Enoplea include both free-living and parasitic forms like whipworms. Secernentea have numerous papillae and lateral excretory canals, and contain many parasitic species of medical importance.

2.  More than 25,000 species. Roundworms.
Nematodes are an extensive group with
worldwide distribution. They include
terrestrial, freshwater, marine, and parasitic
forms. They are elongated roundworms
covered with a flexible, nonliving cuticle.
Circular muscles are lacking in the body wall
and, in Ascaris, longitudinal muscles are
arranged in four groups separated by
epidermal cords (some nematodes have six or
eight groups of longitudinal muscles). Cilia are
completely lacking. roots.

3.  Nematodes are found free-living in almost
every conceivable habitat fromarid deserts to
lake bottoms, rivers, polar seas. Nematodes -
both parasitic and freeliving - are incredibly
abundant. A handful of good garden soil
contains thousands of nematodes. Some 50
different species of nematodes occur in
humans, most of them nonpathogenic. Some
nematodes are plant parasites feeding on plant
sap, especially

4.  1)Bilaterally symmetrical, and vermiform.
2)Body has more than two cell layers, tissues and
organs.
3)Body cavity is a pseudocoel, body fluid under high
pressure.
4)Body possesses a through gut with a subterminal
anus.
5)Body covered in a complex cuticle.
6)Has a nervous system with pharyngeal nerve ring.
7)Has no circulatory system (no blood system)
8)Reproduction normally sexual and gonochoristic.
9)Feed on just about everything.
10)Live just about everywhere, many species are
endoparasites.

5.  Chromadorea
 Enoplea
 Secernentea

6.  The Chromadorea are a class of the roundworm
phylum, Nematoda. They contain a
single subclass (Chromadoria) and several orders.
With such a redundant arrangement, the Chromadoria
are liable to be split up if the orders are found to form
several clades, or abandoned if they are found to
constitute a single radiation.
 Formerly, they were treated as subclass in
the paraphyletic "Adenophorea" assemblage, which
has been by and large abandoned by modern authors.
It is also suspected that the Chromadorea may not
be monophyletic as delimited here; at least
the Monhysterida seem to be a distinct and far more
ancient lineage than the rest.

7.  Members of this class' bodies usually have
annules, their amphids elaborate and
spiral, and they all have three esophageal
glands. They usually live in marine
sediments, although they can live elsewhere.
They have more sophisticated pharynxs than
most roundworms.

8.  Enoplea are a class of the roundworms. Most
are free-living, but the group includes (at least
as delimited here) the order Trichurida, which
includes whipworms andtrichina worms.
 The anterior sensory structures (amphids) are
well developed, while the posterior ones
(phasmids) are not.

9.  Traditionally, the Enoplea were treated as
a subclass Enoplia in the "Adenophorea"
assemblage, alongside the Chromadorea. With
the dismantling of this artificial group, the two
subclasses were upranked to classes of their
own. As it appears, the Enoplea contain two
distinct lineages, as has long been suspected.
These have been reinstated as
subclasses Dorylaimia and Enoplia. But it may
well be that they are just as distant from each
other as from the Chromadorea, necessitating a
further split of the group.

10.  Secernentea are the
main class of nematodes, characterised by
numerous caudal papillae and an excretory
system possessing lateral canals. Like all
nematodes, they have no circulatory or
respiratory system.
 The Secernentea contain several
significant parasitic species among
the Rhabditia and Spiruria. Caenorhabditis
elegans, the famous laboratory animal, is one of
the many harmless members of the Rhabditia.