How are flatworms different from roundworms and segmented worms?

Flatworms, roundworms, and segmented worms are distinct groups of invertebrates with fundamental differences in their body structure and internal organization:

• Body Cavity: Flatworms (phylum Platyhelminthes) are acoelomate, meaning they lack a body cavity and have their internal space filled with spongy tissue called mesenchyme. In contrast, roundworms (phylum Nematoda) are pseudocoelomates with a fluid-filled body cavity that lacks a cellular lining. Segmented worms (phylum Annelida) possess a true coelom, a fluid-filled cavity lined with tissue that is often divided into segments.123456
• Segmentation: Flatworms and roundworms are unsegmented. Segmented worms are characterized by a body divided into successive, repeating segments, which allows for complex movement and specialized organ function.12456
• Circulatory and Respiratory Systems: Flatworms lack specialized circulatory and respiratory systems, relying on their flattened shape and diffusion to transport nutrients and gases. Roundworms also lack an independent vascular system. Most segmented worms possess a well-developed, closed circulatory system to transport materials throughout their bodies.135