Ribbon worms are not segmented. Many are free-living, but parasitic forms are of medical and economic significance. Nematodes are not segmented. The Rotifera are neither segmented nor worm-like. Mollusks do not display segmentation but likely had a segmented ancestor.
Mollusks are coelomate.
The majority of arthropods are insects. Important features are their ability to hang on to their body water and their jointed appendages. Metamorphosis may also account for the insects' great success. Return to Chapter 26. Figure 26.15 clearly shows the body plan of flatworms.