If You Cut An Earthworm In Two, Will The Parts Grow Into Two Separate Worms?

You may have been told as a child that an earthworm will regenerate into two separate worms if it's cut in half crosswise. At least I was.

But I never really had the opportunity to experiment with a real earthworm--all the worms I dug up from my garden ended up as fishing bait.

Like most other animals, the humble earthworm has a head and a tail. Since earthworms have no eyes, the only way to identify its head is by looking at its clitellum.

The clitellum is a saddle-shaped, swollen area that is about 1/3 of the way back on a worm's body. This part, according to wikipedia, functions as the storage area for the eggs of the worm--it secretes mucus to form the cocoon which will hold the worm's embryos. The head of the worm is always located on the end of the worm closest to the clitellum.

See that clitellum there? Image: en.wikipedia.org

When you cut a worm in half, the tail will always die.
The head of the worm may survive and regenerate its tail if the animal is cut behind the clitellum.

But why?

To answer that, we have to look a bit at the anatomy of an earthworm.

Image: Iteachbio.com

It turns out that all the major organs-the brain, esophagus, heart, reproductive system, and etc-of a worm are located within the top half of its body. The bottom half i.e. the tail half contains no vital organs. So the worm can afford to lose its butt and still live to regenerate a new one.

It's not all perfect though. The newly generated part will be slightly smaller in diameter than the original segments and sometimes a hue lighter in color.

Malcolm