Good job! Pikaia is not part of the arthropod lineage. It lacked segments, an exoskeleton, and jointed legs.
When Pikaia‘s fossils were first discovered in the early 1900s, scientists wrote it off as a worm. But in the 1970s, Pikaia was reexamined. Scientists noticed that the rod running along its back resembled a backbone-like structure. Furthermore, the markings on the sides of its body were unusually shaped: they formed V-shapes (pointing towards the head). This zigzag shape is not seen in arthropod segments, but is the typical shape of chordate muscle bundles. Pikaia was probably a chordate — the same group that includes fish, dinosaurs, and humans!
In 1999, researchers discovered fossil chordates in rocks in China that are 10 million years older than Pikaia! Chordates have been around for a long time — since at least the very early Cambrian. Some chordates evolved skulls and a bony spinal column, becoming the lineage known as vertebrates — of which humans are a small part.