Zoology 10th Edition by Stephen Miller,John Harley

Exercise 3

The phylum Arthropoda includes crayfish, lobsters, spiders, insects, and others. Arthropods are characterized by metamerism with tagmatization, a chitinous exoskeleton, and paired jointed appendages. As ecdysozoans, arthropods are most closely related to the nematodes, nematomorphs, and other animals that shed a cuticle during growth. Five subphyla of arthropods have been described.
Why is metamerism, which is common to the Annelida and Arthropoda, no longer considered to be evidence of close evolutionary ties between the two phyla?