is a cuticle as its body covering. Cuticle is a hard protein layer predominantly found in invertebrates such as arthropods, molluscs, echinoderms and nematodes. Unlike a typical cuticle, the cuticle of Pikaia does not have hard extracellular (exoskeleton) protection, and the entirely body is essentially soft-bodied. 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. Pikaia was an active and free swimmer. 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. Reinterpretations Walcott's original summary of the description of Pikaia reads:Body elongate, slender, and tapering at each end. It is formed of many segments that are defined by strong annular shiny lines. Head small with two large eyes and two tentacles... Back of the head the first five segments carry short parapodia that appear to be divided into two parts. The enteric canal extends from end to end without change in character... This was one of the active, free-swimming annelids that suggest the Nephthydidae of the Polychaeta.Whittington and Conway Morris were the first to realise that Walcott's description and classification were not reliable and mostly inaccurate. They compared the body segments as described by Walcott with living animals and found that they were similar to the muscle bundles of chordates such as the living Amphioxus (Branchiostoma) as well as fishes, and not to superficial segments of annelids. They pictured that the muscles would be essential for swimming in water in wriggling motions. The enteric canal as observed by Walcott was not an ordinary digestive tract, it runs along with a stiff rod that resembles notochord. They reported in 1979: "Although Pikaia differs from Amphioxus, in several important respects, the conclusion is that it is not a worm but a chordate appears inescapable." Conway Morris