Worm (animal)

Worm (animal), any soft-bodied animal, usually small and often elongated lacking well-developed limbs. The term does not refer to any particular animal group, but is applied to many unrelated invertebrates or their larvae and to a few vertebrates. The major groups are discussed here.

The familiar earthworm burrows in soil and feeds on dead materials, extracting organic matter from the soil. This moderately complex animal has a complete digestive tract and a circulatory system.

The flatworms are simpler animals that lack an intestine, an anus, and a circulatory system. Some flatworms are free-living and occur in the sea, fresh water, and moist land areas. A familiar example is the freshwater planarian, which crawls about and feeds on small animals. Others, such as the tapeworm and fluke, live inside other animals. The bodies of these parasites tend to be degenerate, or simpler in form than their free-living relatives, in all parts except the reproductive system.

The roundworms are inconspicuous but common. Many roundworms—the pinworm and the hookworm, for example—are parasites of plants and animals; some cause major health problems such as trichinosis and elephantiasis.

Many worms occur only in the sea. The acorn worm has features that suggest a relationship to the chordate lineage. The arrowworm is a peculiar creature that feeds on small animals in the open water and is often abundant. The peanut worm is a distant cousin of the earthworm, and the ribbon worm is related to flatworms. Tube worms belong to various groups and often feed with tentacles.

Some marine worms belong to a category of organisms that scientists call extremophiles because of their ability to withstand extreme conditions once thought to be uninhabitable. The Pompeii worm, for example, lives in scalding water at the mouths of hydrothermal vents on the ocean floor. Pompeii worms keep one end of their two-inch bodies in scalding water at 80° C (176° F), while the other end extends to far cooler water, at 20° C (68° F). This represents a 60° C (108° F) one-end-to-the-other temperature gradient, a condition that no other known animal can regularly withstand. Scientists have also discovered worms more than 550 m (1800 ft) beneath the ocean surface living on and in a freezing crystalline network of methane gas and water, called methane hydrate, that has seeped from beneath the ocean floor.

Scientific classification: Earthworms belong to the phylum Annelida. Flatworms make up the phylum Platyhelminthes. Roundworms make up the phylum Nematoda.