Any of various elongated limbless invertebrates belonging to several phyla. Worms include the flatworms, such as flukes and tapeworms; the roundworms or nematodes, such as the eelworm and the hookworm; the marine ribbon worms or nemerteans; and the segmented worms or annelids.

The New Zealand flatworm Artioposthia triangulata, 15 cm/6 in long and weighing 2 g/0.07 oz, had by 1990 colonized every county of Northern Ireland and parts of Scotland. It can eat an earthworm in 30 minutes and so destroys soil fertility.

In 1979, giant sea worms about 3 m/10 ft long, living within tubes created by their own excretions, were discovered in hydrothermal vents 2,450 m/8,000 ft beneath the Pacific northeast of the Galápagos Islands.

A new species of polychaete worm was discovered in 1997 on the floor of the Gulf of Mexico. It is a pink centipedelike worm that is about 5 cm/2 in long and lives in highly populated colonies in methane ice.