Phylum Nematodes:

The nematodes are known as round worms or Nemathelminthes.

The organisms of this phylum are distributed over wide range of species ranging from 16,000 to 500,000 including round worms and also insects.

Habitat:
The nematodes are nearly adopted to every environment from marine to fresh water soils. They are found in every part of earth lithosphere.

Characteristic of nematodes:

• Triploblastic, vermiform, unsegmented pseu2,do coelomates.
• Body round in cross section and covered by a layered cuticle.
• Complete digestive tract, mouth usually surrounded by lips bearing sense organs.
• Most with unique excretory system comprised of one or two renette cells or a set of collecting tubules.
• Body wall has only longitudinal muscles.

Symmetry:
The animals include in this group has bilateral symmetry

Bilateral symmetry:

The symmetry of being divisible in two equal halves is called bilateral symmetry. Nematodes have bilateral symmetry.

Body shape:

The animals included in this Phylum have unsegmented body plan. There is no segmentation in the body. The body of a typical nematode is slender, elongate, cylindrical and tapered at both ends.

Body organization:
The organisms in this phylum has triploblastic organization.

Triploblastic organization:

The triploblastic animals are made up of three layers i.e. ectoderm, endoderm and mesoderm. The cells of these organisms show greater degree of specialization. They have specialized organs and organ system.

The animals present in this group are pseudo coelomates.  They contain false coelom.

Pseudo Coelom:

It is a cavity derived from the hollow space, the blastocoel, situated in the blastula, not from the mesoderm.

Pseudo coelom in nematodes:
The pseudo coelom in nematodes is a spacious fluid filled cavity that contains the visceral organs and forms a hydrostatic skeleton.

Size:

The size of nematodes typically ranges from microscopic to several meter long.

Locomotion:

The longitudinal muscles are the principle means of locomotion in nematodes. Contraction of these muscles results in the undulatory waves that pass from the anterior to posterior end of the animal creating characteristic thrashing movements. Circular muscles are absent, so bending is dorso ventral.

Mode of life:

Nematodes mostly are endoparasites. They reproduce inside the human body and cause number of harmful diseases.

Examples:

• Ascaris lumbricoides
• Entrobius vermicularis
• Necatar americanus
• Ancylostoma duodenale

Ascaris lumbricoides:

Ascaris lumbricoides is the "large roundworm" of humans, growing to a length of up to 35 cm (14 in).it is one of several species of Ascaris. An Ascaris is a nematode of the phylum nematoda, it is the most common parasitic worm in humans.

Characteristics of Ascaris lumbricoides:

• The adult Ascaris worm is creamy to pinkish-white in color. It's cylindrical in shape with tapering at both ends of the worm.
• Ascaris lumbricoides uses feeds in the human intestine liquid contents.
• Produces a number of diseases in human.
• Major human parasites.

Digestive system of Ascaris lumbricoides:
The digestive system of nematodes is tube like. There are two separate openings called mouth and annus. Nematodes have a complete digestive system consisting of a mouth, which may have teeth, jaws, or stylets, buccul cavity muscular pharynx, long tubular intestine, short rectum and annus.

Hydrostatic pressure in the pseudosoelom and the pumping action on the pharynx push food through the alimentary canal.

The digestive system of nematodes is also tube within tube.

Excretory system of Ascaris lumbricoides:
The excretory system of Ascaris consists of 2 longitudinal excretory canals. They consists of renette cells. They contain renette glands which excrete salts The excretory system of nematoda is also called H system.

Nervous system:
The nervous system of Ascaris  is centralized. It consists of pharyngeal nerve ring  and 4 longitudinal nerve cords.

Respiratory system:
There is no respiratory system in the Ascaris. Respiration generally takes place through skin.

Circulatory system:

There is no circulatory system in the Ascaris.

Reproduction in nematodes:

Most nematodes are diocious, with the males being smaller than the females. The gonads lie free in the pseudocoelom. Here we are going to describe the reproduction in Ascaris lumbricoides.

Female reproductive system:
The female reproductive system consists of a pair of convoluted ovaries. Each oviduct becomes tubular uterus; the two uteri unite to form a vagina that opens outside through a genital pore.

Male reproductive system:

The male system consists of a single testis which is continuous with a vas deferens Males are commonly armed with a posterior flap of tissues called a bursa. The bursa aids the male in the transfer of sperms to the female genital pore.

Life cycle of Ascaris lumbricoides:

Life cycle of Ascaris lumbricoides consists of following events.
After copulation, hydrostatic forces in the pseudocoelom move each fertilized egg to the gonopore. The number of eggs produced varies with the species; some nematodes produce only several hundred eggs. Some nematodes give birth to larvae that are called ovoviviparity.
External factors such as temperature and moisture influence the development and hatching of eggs. Hatching produces a large number of larvae. The larvae undergo four molts although in some species, the first one or two molts may occur before the eggs hatch.
Important parasites of phylum nematodes:
Following are the important parasites of phylum nematoda:

• Ascaris lumbricoides: (giant round worm)

it is found in human small intestine heart and lungs and cause diseases such as bloody sputum, cough , fever, and abdominal discomfort.
Entrobius vermicularis: (pin worm)
it is found in human large intestine and causes itching of anus, inflammation of mucous membrane , insomnia and loss of appetite.
Ancylostoma duodenale: (hook worm)
It is found in human small intestine and causes anemia, physical and mental retardation.

 Economic importance of nematiodes: 

• The nematodes are considered as major plant parasitic nematodes because they can cause 10-30 %losses reported on citrus trees.
• They also parasitize other hosts such as olive, grape, persimmon and lilac.
• They are major component of soil and sediment ecosystem
• they are also beneficial because they kill deer ticks, which cause Lyme disease.
• They also provide nutrient to the soil.