Propose an explanation for why annelids and arthropods were thought to be closely related

Because of their segmented bodies

Explanation:

Annelida and Arthropoda are two phyla of the kingdom Animalia. Their main similarity is that both of the phyla are protostomes, with segmented bodies (their body systems are also segmented).

The difference between Annelid and Arthropoda is that Annelida consists of a hydrostatic skeleton while Arthropoda consists of an exoskeleton made up of chitin. Also, the body of the Arthropoda is divided into head, thorax and abdomen, while Annelida have no distinct head.