PART 6 of the Burgess Shale Fossils: 

Pikaia gracilens Walcott (1911, pl. 20, figs. 1. 2) is a legless, soft-bodied, ribbon-like animal, up to 2 inches (5 cm) long, whose closest relatives are the sea-lancelet Branchiostoma and the zebrafish. Although Pikaia was originally thought to be an annelid worm, it is not because it is unsegmented. In the 1970s, its remains were restudied and a notochord was discovered = a rod running along its back and resembling a backbone-like structure. In addition, there are markings on the sides of its body, and they resemble myomeres (chordate muscle bundles) found on living chordates (backboned animals). As a result of this discovery, Pikaia is now classified as probably as a basal, or “stem chordate,” ancestral to all back-boned animals (including fish to humans). In the 1970s, it was reported that Pikaia probably swam like an eel, but, in recent years, researchers have concluded that it was probably a slower swimmer and less flexible than an eel.

Key Reference: 

Walcott, C.D. 1911. Cambrian geology and paleontology. II. No. 5. Middle Cambrian annelids. Smithsonian Miscellaneous Collections.