Roundworms reproduce sexually. The female roundworm has an ovary, and she holds eggs in her oviduct until she passes them into her uterus, where they are fertilized by the male. The male produces sperm cells in his testis and stores them in his vas deferens until it is time to reproduce; then he passes them through a spicule to fertilize the female's eggs.
Once fertilized, roundworms can deposit their eggs into the soil to await hatching. More than 200,000 eggs may be deposited into the soil at once.
Roundworms are also called nematodes, and they belong to the phylum Nematoda. There are more than 25,000 species of nematodes known to man, although estimates hold that there may be as many as 1 million species altogether.
The male of the species is usually smaller than the female, and the male's tail is bent slightly, which allows him to hold the female. While copulating, the male inserts his spicules into the female's genital pore.
Around 90 percent of all nematodes reside in the top 15 centimeters of the soil. There, they feed on living material and decomposing organic matter. In fact, nematodes are sometimes introduced into gardens to help increase the nitrogen level of soil.