Final answer:
The synapomorphy for the phylum Annelida is metamerism or true segmentation.
Step-by-step explanation:
The synapomorphy for the phylum Annelida is metamerism or true segmentation. Annelids, such as earthworms and leeches, have a segmented body plan with repetition of internal and external features in each body segment. This allows for the evolution of independent modifications in different segments that perform different functions.