Worms are arranged in four major groupings, or phyla:  Flatworms (Platyhelminthes), Ribbon worms (Nemertea), Roundworms (Nematoda), and Segmented worms (Annelida). 

The common earthworm is an annelid.  Its body is divided into about 150 ring-like segments, and ranges from nine to ten inches in length.  An earthworm, as its name implies, inhabits the soil, where it processes organic matter through its body, thus releasing nutrients and enriching the environment. 

Mature earthworms are hermaphrodites, possessing both female and male organs (though the worms do not self-fertilize).  The swollen band of tissue found on these worms is a sexual organ called the clitellum.  The worms secrete a mucus band from this spot which serves to combine eggs and sperm received from another worm.

An earthworm moves by contracting its longitudinal and circular bands of muscles, resulting in a crawling motion.  It is through the skin of the worm that oxygen is absorbed and carbon dioxide released.  For this reason the skin must remain slimy and moist in order for the earthworm to breathe.