part of the sides; the front; and the bottom. Most of the sclerites were shaped like oval leaves, but the ventro-lateral ones, nearest the sea-floor, were crescent-shaped, rather like flattened bananas, and formed a single row. Larger specimens (>~15mm) bear two rows of ribbed spines running from front to rear, one along each side of the top surface, and projecting out and slightly upward, with a slight upward curve near the tips. Although the spines in the middle of each row are usually the longest, up to long, a few specimens have rather short middle spines that represent part-grown replacements. Each sclerite was rooted separately in the body; the roots of body sclerites are 40% of the external length or a little less, while the roots of the spines are a little over 25% of the external length; all were rooted in pockets in the skin, rather like the follicles of mammalian hair. The roots of the body sclerites were significantly narrower than the sclerites, but the spines had roots about as wide as their bases; both types of root were made of fairly soft tissue. They bore protrusive, presumably structural, ribs on their upper and (seemingly) lower surfaces. The sclerites and spines were not mineralized, but made of a tough organic (carbon-based) biopolymer. Butterfield (1990) examined some sclerites under both optical and scanning electron microscopes and concluded that they were not hollow, and that the bases split and spread to form the blades, a pattern that is also seen in monocot leaves. The sclerites bear an internal fabric of longitudinal chambers, which suggest that they were secreted from their bases in the manner of Lophotrochozoan sclerites. As known from Marrella and Canadia, its sclerites may have been iridescent due to evidence of diffraction grating, although questioned in later study. Ontogeny Wiwaxias scleritome comprises eight rows of sclerites, arranged in bundles. Sclerites are periodically shed and replaced during growth, with the number of sclerites in a given bundle increasing as the animal ages to produce a thicker scleritome. Once specimens reach a certain size, spines are added to the scleritome; this size is ~15 mm in W. corrugata but substantially smaller in W. taijengensis. One juvenile specimen was originally interpreted as molting, but in fact represents a single, folded, individual. Ecology The long dorsal spines may have been a defense against predators. Wiwaxia apparently moved by contractions of a slug-like