There are about 9,000 species of Annelids known today, including worms and leeches. They can be found almost anywhere in the world. Annelids have existed on Earth for over 120 million years.

Annelids have bodies that are divided into segments. They have very well-developed internal organs. One common characteristic of annelids is that they don't have any limbs.
Some annelids may have long bristles. Others have shorter bristles and seem smooth, like the earthworm shown here.
There are many types of worms. Commonly known worms include earthworms, roundworms and flatworms. Most worms are small, measuring fractions of an inch to several inches long. Other worms, such as the ribbon worm, can grow up to 100 feet in length. Some worms are considered parasites, in that they live inside the human body.