Although primitive, "Pikaia" shows the essential prerequisites for vertebrates. When alive, "Pikaia" was a compressed, leaf-shaped animal with an expanded tail fin; the flattened body is divided into pairs of segmented muscle blocks, seen as faint vertical lines. The muscles lie on either side of a flexible structure resembling a rod that runs from the tip of the head to the tip of the tail. It likely swam by throwing its body into a series of S-shaped, zigzag curves, similar to the movement of eels; fish inherited the same swimming movement, but they generally have stiffer backbones. These adaptations may have allowed "Pikaia" to filter particles from the water as it swam along. "Pikaia" was probably a slow swimmer, since it lacked the fast-twitch fibers that are associated with rapid swimming in modern chordates.