Earthworms produce cocoons in which their young ones or embryos develop.
Earthworms live in soil. They dig burrows and make soil porous. Their excreta make the soil rich with natural manure. Earthworms are called friends of farmers.
The body of earthworm is long and segmented. On the skin there are fine bristles or setae which provide grip. By stretching and contracting its body, the worm moves on the soil or in the underground tunnels.
When the earthworm becomes sexually mature, a saddle shaped structure becomes conspicuous on its body. It is situated about 1/3 the distance away from head. It is called Clitellum. It contains glands which produce mucous, a slimy fluid.
Earthworms are hermaphroditic. Each worm has both male and female sex organs. During mating two worms attach to each other, exchange sperm. And separate.
The clitellum plays an important role in reproduction. Its glands produce mucous which becomes a sheath containing nutritive material. The sheath slides forwards, collecting eggs and sperm from the genital openings of the worm. The sheath finally slides off from the head of the worm. As it separates from the worm, its ends are sealed. It now becomes a cocoon.
The worm deposits the cocoon in the soil. The cover of the cocoon soon becomes thick and protects the eggs and sperm.
The eggs and sperm fuse, resulting in embryos. The embryos grow inside the cocoon, absorbing the stored nutritive material.
Earthworms produce between 4 and 70 cocoons per year. Those worms which live deep in the soil produce less number of cocoons, while the worms living on the upper layers produce more. This is because; danger from drought and predators is more on the upper layers.
Each cocoon may contain two to twenty embryos. Depending upon favorable conditions, young worms hatch from cocoons. They mature in ten to fifty weeks to produce cocoons over again.