Propose an explanation for why annelids and arthropods were thought to be closely related, before phylogenetic analyses in the late 1990s. Propose an explanation for why annelids and arthropods were thought to be closely related, before phylogenetic analyses in the late 1990s. They have unsegmented bodies, unlike other protostomes. They have segmented bodies, unlike other protostomes. They have exoskeleton, unlike other protostomes. They are round worms, unlike other protostomes.