The phylum Nematoda includes roundworms, one of the most common phyla of animals, with over 20,000 different described species (over 15,000 are parasitic) and are found in all habitats.

They are bilaterally symmetrical and unsegmented. Roundworms are cylindrical in shape, tapered at both ends, and vary in length from being microscopic to 20 inches long.

 

Roundworm

 

Unlike flatworms, roundworms have a complete gut. This means that they have a gut that begins with a mouth and ends with an anus.

They have separate sexes generally and reproduce sexually.

Most roundworms are parasites. The roundworm Trichinella, commonly known as the trichina worm, occurs in rats, pigs, and humans, and is responsible for the disease trichinosis. People get trichinosis from eating infected, undercooked pork. Trichinosis can affect the heart.

Another roundworm, Ascaris, parasitizes human lungs.

The Filaria worm attacks the lymphatic system causing great swelling.

Hookworms and pinworms are also roundworms which parasitize humans. The best known is the human pinworm, also known as the threadworms Enterobius vermicularis. The pinworm lives in the lower part of the small intestine, and the upper part of the colon. It is found worldwide and causes the common infection enterobiasis in humans.

Filaria worms