Adult animals that have soft, elongated, often tubelike bodies and that lack backbones are commonly called worms. Worms are so different from one another that zoologists do not classify them together in a single group; they place them in about a dozen different and often unrelated taxonomic groups called phyla. In everyday language the name worm may be loosely applied to other animals as well to the larvae, or immature forms, of some insects, for example, or even to some vertebrates, such as the blindworm, a limbless, snakelike lizard. However, the name is properly applied only to certain adult invertebrates.
Worms play a major role in virtually all ecosystems. Some terrestrial worms condition the soil. Many worms are parasites of plants and animals, including humans. Many free-living, or nonparasitic, worms form an important link in food chains.
All worms are bilaterally symmetrical, meaning that the two sides of their bodies are identical.
They lack scales and true limbs, though they may have appendages such as fins and bristles. Many worms have sense organs to detect chemical changes in their environments, and some have light-sensing organs.
Different groups of worms may have significantly different internal characteristics. Flatworms, for example, lack a coelom a cavity in the body between the internal organs and the body wall. They may also lack a digestive tract, or the digestive tract may be greatly simplified. Proboscis worms also lack a coelom, but they are more advanced than flatworms, in terms of evolution, because they have a complete gut with a separate mouth and anus and a true circulatory system. Roundworms have a complete digestive system and a pseudocoel, or false coelom. Segmented worms are the most advanced because they have a true coelom and well-developed digestive, respiratory, circulatory, and nervous systems.
Some worms are...