Final answer:
Pinworms are roundworms from the phylum Nematoda, characterized by smooth bodies, bilateral symmetry, and a complete digestive system.
Step-by-step explanation:
Pinworms are a type of roundworm. They belong to the phylum Nematoda. Roundworms are characterized by having smooth bodies, bilateral symmetry, and a complete digestive tract that includes separate mouth and anus openings. This is in contrast to flatworms of the phylum Platyhelminthes, which often have an incomplete digestive system. Additionally, the body of roundworms is round due to the presence of a pseudocoelom, while flatworms have a flattened body.