It is a common misconception that an earthworm becomes two worms when cut in half. However, only a limited number of earthworm species. are capable of anterior regeneration. When such earthworms are bisected, only the front half of the worm (where the mouth is located) can feed and survive, while the other half dies. Species of the planaria family of flatworms actually do become two new planaria when bisected or split down the middle.

From a total of around 6,000 species, only about 150 species are widely distributed around the world. These are the peregrine or cosmopolitan earthworms.

An earthworm is a tube-shaped, segmented animal commonly found living in soil, that feeds on live and dead organic matter. Its digestive system runs through the length of its body. It conducts respiration through its skin. An earthworm has a double transport system composed of coelomic fluid that moves within the fluid-filled coelom and a simple, closed blood circulatory system. It has a central and a peripheral nervous system.

The central nervous system consists of two ganglia above the mouth, one on either side, connected to a nerve cord running back along its length to motor neurons and sensory cells in each segment. Large numbers of chemoreceptors are concentrated near its mouth. Circumferential and longitudinal muscles on the periphery of each segment enable the worm to move. Similar sets of muscles line the gut, and their actions move the digesting food toward the worm's anus.

Earthworms are hermaphrodites - each individual carries both male and female sex organs. As an invertebrate, it lacks a skeleton, but it maintains its structure with fluid-filled coelom chambers that function as a hydrostatic skeleton.