Nematodes, known as a roundworm, is phylum in the Kingdom Animalia of the biological classification system known as the Linnaean Taxonomic Hierarchy. Roundworms are one of the most common phyla of animals, this phyla include more than 20,000 species (over 15,000 are parasitic). Only arthropods are more diverse than roundworms. Roundworms look alike earthworms, but they are completely different. They live in places where they outnumber other animals in both individual and species counts such as freshwater, marine, and terrestrial environments. Roundworms are found all the way from Antarctica to North Pole,  but they are usually willing to live in warm areas. There are many kinds of parasitic forms in roundworms. The hosts in which they live include most plants and animals, and also humans. Adult roundworms usually range from 6 to 20 inches in length and 1/10 to 2/10 inches in diameter. 
Roundworms are one of the simplest animal groups that have a complete digestive system. Since roundworms have no respiratory or circulatory systems, they use diffusion for respiration and circulation of substances around their body. Even though they are thin and round, they are actually bilaterally symmetric. Roundworm’s body cavity is a pseudocoel, which lacks the muscle, so that roundworms have to depend on body movement and internal/external pressure to force food down through their digestive tract.
Roundworms are surrounded by a strong, flexible noncellular layer called a cuticle. Cuticles made of keratin are secreted by and cover a layer of epidermal cells, which protects the body from drying out, or from other harsh environments. These cuticles don't allow the volume of worm to increase, to keep hydrostatic pressure inside the worm high. Therefore, when the worm grows, it has to molt and form new cuticles. This makes the roundworm unnecessary to possess circular muscles. This hydrostatic pressure is the reason why the roundworms are round.
Roundworms reproduce sexually. Males are usually much smaller than females and have bent tail to hold the female while copulation. Females have ovaries for holding eggs in oviducts and then pass eggs to the uterus where they are fertilized. During the copulation, male protrudes one or more spicules out of the cloaca and insert them into genital pore of the female. Amoeboid sperm cells are passed along the spicules into the female worm. Eggs can be either embryonated or unembryonated when passed by the female, which means that their fertilized eggs are not yet developed. In free-living roundworms, the eggs hatch into larva, which eventually grow into adults; in parasitic roundworms, they usually born in host’s intestine and grow there. 
Roundworms are known to be found in virtually every habitat from Antarctica to oceanic trenches, but they are usually willing to live in warm area. Most roundworms live in the spaces between soil particles, or other substratum. According to the report, 236 species of roundworm were living in a few cubic centimeters of mud. Their ecology depends on such physical properties as viscosity, gaseous diffusion, surface tension, water percolation, and humidity. 
Although some of roundworms are filter feeders, most of them generally eat bacteria, fungi and protozoan. Excretion is processed through an anus at roundworm's rear end or a series of excretory tubes that end in an excretory pore. 
Roundworm eggs can enter the body when we are eating foods or drinking water, touching contaminated soil with open wound , moving unwashed hands that touched contaminated soil to the mouth.
When infected by the roundworm, these symptoms happen; irritability, restlessness at night, erratic or poor appetite, constant fatigue, weight loss, abdominal discomfort, diarrhea, fever, coughing, and wheezing. Sometimes, these parasite roundworms can be found in the stool or in bed.
Doctors can prescribe drugs that kill the worms. About a week is needed for complete recovery. The medications can't be used to people who are in pregnancy, because medications can harm the baby. Care from a health care professional is needed to completely eradicate the problem.
If people who are infected left untreated, roundworms can move to other body parts and could possibly cause intestinal obstruction. 
A human intestinal roundworm
- Nematoda by Wikipedia
- Roundworm by Animals
- Roundworm infection by health square
- Roundworm reproduction by Answers.com
- Roundworm ecology by Seattle university