The Nematodes are known as roundworms. Most are free-living but some are parasites. These organisms are ubiquitous pseudocoelomates. Parasitic forms that affect our pets include the heartworms. Parasitic roundworms that affect us include Enterobius_vermicularis, Ascaris_lumbricoides, and Trichinella_spirilis.

Please observe that the male Ascarid worm has a hooked posterior end. The sex of an Ascarid worm can be determined with this simple external observation.

The cross-section of a male Ascarid worm, Ascaris lumbricoides, shows the  protective cuticle, as well as the pseudocoelomate_cavity. The loops of testes, seminal_vesicles and intestines are clearly visible.

The cross-section of a female Ascarid worm, Ascaris lumbricoides, shows the  protective cuticle, as well as the multiple loops of ovaries, the intestine, the pseudocoelomate cavity, and the distinctive uteri.