The nematoda are also known as round worms. They were proposed by Gegenbaur.
It acts as a parasite on plants and animals and causes agriculture and health related problems. They are aquatic and also occur in the soil. They arise from the three germ layers such as ectoderm, mesoderm and endoderm. They are known as triploblastic animals. They have no skeletal, respiratory and circulatory system. The space between the body wall and the digestive tract is called pseudocoel. It is filled with the pseudo coelomic fluid and that’s why the flat worms are known as pseudo coelomates. This fluid maintains the body shape. They have bilateral symmetry. They have an organ system level of organization. Their body wall is made up of cuticle, epidermis and muscular layer. The digestive system is complete and has mouth and anus for ingestion and egestion. Their respiration is of aerobic type with the diffusion of gases by the body surface. The mode of excretion is by the gland cells or canals with ammonia as their main excretory product. The nervous system is ring like and gives rise to nerves. They have direct development and also have sensory organs. The sensory organs include papillae and amphids. These are present on the lips and are gustatory in function. There are chemo receptors present on the lateral side of the body known as phasmids. Mostly the round worms show sexual dimorphism and are unisexual. They have the internal method of fertilization and there is absence of asexual reproduction.
The nematodes are better than the platy helminthes as they have pseudocoel, unisexual condition and a fully formed digestive tract.
The rhabditis are roundworms which may be free living or occur as a parasite. It lives in the soil with the small adult male. The male has one testes and the female has couple of ovaries.