| Flatworms are unsegmented, bilaterally symmetrical worms that lack a coelom but that do have three germ layers. |
| To gain a more accurate understanding of the segmented or unsegmented nature of these four taxa, two types of data must be gathered. |
| Phylum Nemertea contains about 1,150 species of unsegmented worms that possess an eversible proboscis contained in a fluid-filled cavity or rhynchocoel. |
| These unsegmented worms have been found in Cambrian rocks, such as the legendary Burgess Shale. |
| Moreover, this rod was unsegmented, for the notochord is devoid of segmentation. |
| Sipunculans, or peanut worms, have an unsegmented body with a retractable introvert. |
