Final answer:
Annelids are most closely related to molluscs, sharing the Lophotrochozoa clade within the protostomes. They both exhibit bilateral symmetry and a triploblastic body, with a common evolutionary ancestor demonstrated by molecular evidence and developmental similarities.
Step-by-step explanation:
Annelids are most closely related to molluscs.
Annelids and molluscs are both within the Lophotrochozoa clade of the protostomes, which is one of the major divisions in the animal kingdom based on embryonic development. They share a common ancestor and have similar developmental stages. While it was once thought that annelids and arthropods were closely related due to their segmented body plans, molecular evidence has shown that arthropods are actually more closely related to nematodes. This is because they belong to the Ecdysozoa clade and share the process of ecdysis, or molting.
Annelids and molluscs both exhibit features such as bilaterally symmetrical bodies and being triploblastic. They differ in that molluscs generally have a ventral muscular foot and may secrete a calcareous shell, while annelids are characterized by their segmented bodies and the presence of chaetae, or bristle-like structures. Despite these differences, the similarities in their organ systems and developmental stages point towards a closer evolutionary relationship between annelids and molluscs.