The clitellum produces a mucous sheath and nutritive material, and as the sheath slides forward, it picks up ova from the earthworm's ovaries then packets of sperm that had been transferred to the worm from another worm during mating. As the clitellum sheath slides off the worm's head, the ends are sealed to form the cocoon.
Young worms hatch from their cocoons in three weeks to five months. The gestation period varies for different species of worms. It also depends on conditions like temperature and soil moisture. Hatching is delayed if conditions are poor, and cocoons may overwinter in soil to hatch in the spring.
Each cocoon holds from one to twenty fertilized ova or eggs-depending on the species and also nutrition of the adults laying them and environmental conditions like soil moisture. Usually only a few to several young worms successfully emerge from each cocoon.
Earthworms can produce between 3 and 80 cocoons per year depending on the species. The deeper-dwelling species don't have to produce as many cocoons because they are protected much better from predation. Surface-dwelling species tend to produce many more cocoons. Worms mature in 10 - 55 weeks, depending on the species. Certain species could live 4-8 years. It depends on predators and environmental conditions. There seems to be some debate among worm "experts" on the length of a worm's life. I found other information suggesting worms may live up to 15 years again depending upon environmental conditions and species type.
The head of the worm is always located on the end of the worm closest to the clitellum. Place a worm on a rough piece of paper and observe which direction it travels. Earthworms usually extend their "head" first when crawling. The band closest to the head is the clitellum. This saddle-shaped, swollen area is about 1/3 of the way back on a worm's body. The clitellum secretes mucus to form the cocoon which will hold the worm embryos. The presence of the clitellum is the sign of a sexually mature worm.
Here is more interesting information about the common earthworm.
Lumbricus terrestris (Nightcrawler, Dew worm) is one of North America's largest earthworm species. It ranges in size from 9-30 cm with a diameter of 6-10 mm. A nightcrawler nearly 30 cm long (stretched out) and weighing 11.2 g was collected in a soybean field in Ontario, Canada. The largest tropical species are up to 120 cm long. The largest in the world are some Australian forms which may reach 300 cm in length. I want me one of those! My compost would be incredible.
Earthworms have bristles or setae in groups around or under their body. The bristles, paired in groups on each segment, can be moved in and out to grip the ground or the walls of a burrow. Worms travel through underground tunnels or move on the soil surface by using their bristles as anchors, and pushing themselves forward or backward using strong stretching and contracting muscles and can move both frontward and backward they tend to travel forward more.
If you cut an earthworm in half,the rear half will always die, and the front half may live to become another whole worm. This is called regeneration. The front part must be long enough to contain the clitellum and at least 10 segments behind the clitellum. This makes up about half the length of the worm. The new posterior segments grown will be slightly smaller in diameter than the original segments and sometimes a bit lighter in color. And they breathe through their skin. They need humid conditions to prevent drying out.
Worms appear slimy because they coat themselves in mucus, which enables dissolved oxygen to pass into their bloodstream. Worm casts are the earthworm's undigested waste (worm poop). Casts are composed of organic matter mixed with soil. They are held together with "gum" produced many active bacteria and mucus. This is why we gardeners love worms so much, they are so good for our soil.
Excerpts from Thomas J. Barrett's Harnessing the Earthworm(Bruce Humphries: Boston, 1947; copyright unrenewed.) and additional excerpts and picture courtesy of Worm World.
Credit for the idea for this article goes all the Worm Lovers in the DG Soil and Composting Forum, who triggered a conversation on harvesting worms in the rain and it went downhill from there! Thanks guys!