The internal structure of nematodes is quite complex. Although most roundworms are free-living, there are many parasitic forms. These include pinworms , hookworms , trichinella , and heartworms . Even most leeches are carnivores. Roundworms are pseudocoelomate - they have a cavity that forms between the mesoderm and the endoderm.
Segmented worms are coelomate - they have a true coelom which forms within the mesoderm.
Some worms show a great ability to regenerate lost or injured parts, others do not. Some worms reproduce asexually, others sexually, and still others can do both. Earthworms reproduce sexually. The eggs of an earthworm are enclosed for 2 to 3 weeks in a cocoon secreted by the clitellum. The visible measurement units in the picture of earthworm cocoons here are millimeters.