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Pikaia was first described by Walcott as a marine worm. However, because of several of its features, it was included in the phylum Chordata, which also includes vertebrates. The first known Cambrian representative of this phylum, it had a pair of short tentacles on the front. The trunk consisted of solid blocks of muscle, curved in an S shape. The tail expanded in the form of a fin. Pikaia swam just above the sea bed. It propelled itself by using the muscles to undulate its body.78
