A group of various unrelated invertebrates, worms have soft, elongated bodies. Invertebrates are animals that lack a backbone. Worms are so different from each other that they have not been classified under a single group.
Where worms live
There are at least 20,000 kinds of worms. They are found almost all over the world. Some worms that live in the ground help to condition the soil. Many thrive as parasites of plants and animals, including humans. Parasites live in other animals and cause harm. Several other worms may be free-living, or nonparasitic. There are worms that live in freshwater, seawater, and even on the seashore.
Different kinds of worms
In everyday language, the term worm is also applied to various other living forms such as larvae, insects, centipedes, or even some vertebrates (creatures with a backbone) such as blindworms. Worms can be divided into several groups, of which three are important.
- Flatworms come in many shapes and sizes, though most are flattened in appearance.
The first of these includes the flatworms. They have a flat, ribbon- or leaf-shaped body with a pair of eyes at the front. Some are parasites.
The second group contains the threadworms, roundworms, and hookworms. Threadworms may be microscopic, such as the vinegar eelworm, or more than 3 feet (1 meter) long. They are found in damp earth, moss, decaying substances, fresh water, or salt water. Many are parasites. Some roundworms are also parasites. The Guinea worm, for example, gets under the skin of the feet and legs of people living in tropical countries.
- Earthworms belong to a group of worms known as segmented because their bodies are generally divided …
The third group consists of the segmented worms, with bodies divided into segments, or rings. Among these are the earthworms and the bristle worms of the sea.
Characteristics
Worms usually have a cylindrical, flattened, or leaflike body shape and are often without any true limbs. Instead, they may have bristles or fins that help them move. Many worms have sense organs that can detect environmental change. A few may even have light-sensing organs.
Some worms reproduce sexually. Hermaphroditism, the condition in which a single individual possesses both male and female reproductive parts, is common in many groups of worms. Asexual reproduction, whereby new individuals develop from the body cells of another, also occurs in some worms.