Worms are primitive invertebrates. They have no heads, eyes or brains.

Because worms do not have feet and their bodies are soft, they are an easy source of food for many organisms, especially birds. This is why worms mainly live underground. 

The animals that we think of as worms actually belong to many different groups of invertebrates. For example, earthworms are decomposers that live in underground tunnels. They belong to the group of annelids, or segmented worms. 

On the other hand, flatworms live as parasites inside other organisms. Most worms thrive in moist environments.