Round worms are also known as nematodes. Their identifying characteristic is their digestive tube that spans the length of their body. They have a circulatory system and their nervous system consists of two cords of nerve tissue. Some nematodes are poisonous and harmful to the predator if consumed.

There are about 20,000 known species of roundworms.

One example of a roundworm is Caenorhabditis elegans. These worms are only about 1 millimeter long.

Another example is the giant roundworm of humans, also known as Ascaris lumbricoides. This worm infects about one-sixth of the human population, giving them ascariasis.

http://www.biocontrol.entomology.cornell.edu/pathogens/nematodes.html

http://www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html