- Some earthworm species seem better-capable of regeneration than others. In their book "Biology and Ecology of Earthworms," authors C.A. Edwards and P.J. Bohlen state that scientists believe worms can more readily regenerate an amputated tail than an amputated head, and there seems to be a limit to how many times a given worm can regenerate portions of its body.
- Edwards and Bohlen also note that a head regeneration typically grows at the same width as the rest of the body, and a regenerating tail starts more slender and does not fill out until all its segments have been reproduced.
- Individual earthworms possess both male and female sex organs--they are hermaphroditic. Some species nonetheless mate, with both participating worms fertilizing each other at the same time. Other kinds of worms reproduce asexually without mating.