Ontogeny
Wiwaxia grew by expansion rather than addition; that is to say it appears to have maintained a constant complement of scales as it grew, with each sclerite growing larger (by an unspecified means, probably by molting and replacement) and changing slightly in shape, but no new sclerites being inserted. 16–18 ventro-lateral sclerites are present on each side of a specimen, and the dorsal sclerites are arranged in 7–9 rows; the variation in number may be genuine but could reflect partial preservation. Its length:width ratio was constant through growth (although spineless "juveniles" may have been slightly flatter).
One juvenile specimen appears to be preserved while molting and not yet completely detached from its discarded armor. Its new set of spines seem less rigid than the old ones and slightly underdeveloped, as if the next stages were going to be inflated by body fluids and then hardening. The new armor may have had an internal volume 50% to 70% larger than the old one. Molting appears to have occurred all at once, as adult specimens shows no signs of interruptions in the sclerite armor that would indicate molting of parts of the armor or of individual sclerites. Since the bases of the body sclerites are relatively narrow and these is no sign of sclerites splitting during molting, withdrawing soft tissue from the old sclerites would probably have required the tissues to be broken down in to a more fluid form, as happens in the claws of lobsters and crabs when they molt. The skin must also have been shed, since the discarded armor appears as a complete unit rather than scattered sclerites. In the juvenile that was apparently molting when it died, the feeding apparatus also appears to have been shed, as half of one tooth row is pointing forwards.
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