A special organ, the clitellum , secretes a mucous cocoon which slides along the worm, picking up its eggs and then the stored sperm. The cocoon slips off the worm into the soil and protects the embryos while they develop. Asexual reproduction occurs in some species by fragmentation followed by regeneration. Movement involves coordinating longitudinal and circular muscles in each segment with the fluid-filled coelom functioning as a hydrostatic skeleton. Waves of alternating contractions pass down the body.
Most aquatic annelids are bottom-dwellers that burrow, although some swim in pursuit of food.
The Class Oligochaeta contains earthworms and a variety of aquatic species. Important to farmers as they till the soil and castings improve soil texture. Charles Darwin estimated that 1 acre of British farmland had about 50,000 earthworms that produced 18 tons of castings per year. The Class Polychaeta contains mostly marine species. A few drift and swim in the plankton, some crawl along the sea floor, and many live in tubes they construct by mixing sand and shell bits with mucus.