Final answer:
Roundworms have a body cavity and a complete digestive system, while tapeworms have a segmented body and lack a digestive system.
Step-by-step explanation:
The main difference between roundworms and tapeworms is their body structure and digestive system.
Roundworms have a body cavity with internal organs, while tapeworms have a segmented body made up of proglottids.
Roundworms have a complete digestive system with a mouth, anus, and gut, whereas tapeworms lack a digestive system and absorb nutrients directly from the host's digestive system.