Phylum Nematoda includes unsegmented, triploblastic, pseudocoelomate, cylindrical, rounded vermifora organisms with glistening smooth surfaces and a tube-within-a-tube body plan. Nematodes are bilaterally symmetrical, elongate, and generally elongated at both ends. Some species possess a pseudocoel, a fluid-filled body cavity between the digestive tract and the body wall. About 15,000 species are known. The members show a little structural advancement over Platyhelminthes in having irrelevant body cavity and a complete alimentary canal. Like arthropods and members of six other phyla, nematodes secrete an external cuticle that is periodically molted. These animals have been provisionally grouped together as the Ecdysozoa, a taxonomic category based on the assumption that molting has evolved only once.

The sexes are distinct in most species, but some are have both male and female reproductive organs in the same individual. Nematodes variety in size from microscopic to 7 meters long, the largest being the parasitic forms found in whales.
Nematoda parasite of animals occur in practically all organs of the body, but the most common sites are in the alimentary, circulatory, and respiratory systems. Some of these worms are known by such common names such pinworm, hookworm, threadworm, lungworm etc.

Nematoda Roundworms

Nematoda can cause a variation of diseases like filariasis, ascariasis, and trichinosis and parasitize several crop plants and domestic animals.
Phylum Nematoda is characterized by the following features.
(1) Free-living in fresh and marine waters and also on soil (e.g., Rhabditis), or endoparasitic in animals and plants.
(2) Body unsegmented, elongated, cylindrical and vermiform with narrowing ends; anterior end shows a degree of cephalization.
(3). Body triploblastic, bilaterally symmetrical with a pseudocode body cavity.
(4) Digestive tract is a straight tube with a terminal mouth; anus also present.

(5) Circulatory and respiratory system absent.
(6) Respiration made through general body surface.
(7) Simple excretory system, consisting of canals or gland like organs; flame cells are absent.
(8) Nervous system consist of a nerve ring and anterior and posterior nerve cords.
(9) Sense organs in the form of papillae and amphids.
(10) ) Reproductive system of nematode well developed; usually unisexual with sexual dimorphism; males are smaller than females.
(11) Fertilization internal and development naturally direct.
(12) ) Life cycle of nematoda complex with usually four larval stages.