Final answer:
Organisms in Phylum Nematoda, or roundworms, have a pseudocoelomate body organization with a complete digestive system and a body cavity known as a pseudocoel. They show bilateral symmetry and have both free-living and parasitic forms.
Step-by-step explanation:
Organisms in the Phylum Nematoda, commonly known as roundworms, have a pseudocoelomate organization. Nematodes differ from flatworms as they have a complete digestive system with a mouth and an anus, and a body cavity known as a pseudocoel, unlike the acoelomate body plan of flatworms. They exhibit bilateral symmetry, meaning they have symmetrical right and left sides when split lengthwise. This distinguishes them from flatworms, which are acoelomate, meaning they lack a true body cavity. Nematodes include both free-living and parasitic species. They have a complete digestive system and undergo molting to increase in size.
Features such as a simple excretory system, ecdysis (shedding of the cuticle), and a hydrostatic skeleton are typical within the phylum. Nematodes can be free-living in various habitats or parasitic, infesting plants, animals, and humans. Embryonic development occurs externally with several larval molting stages. The most researched free-living nematode, Caenorhabditis elegans, has contributed greatly to our understanding of molecular and developmental biology.