With the hard exoskeleton being shed periodically and a larger new one being developed. After each moulting the trilobite was only covered by a flexible, chitinous shell which gradually mineralized and hardened. Gradual separation of the cephalon from the transitory pygidium occurs. The overall size of the trilobite increase up to more than ten times the size when in the Protaspid period. This period terminates when the trilobite has grown the number of thoracic segments characteristic of that species at adulthood.
Growth is continuous through several moultings.
Changes occur in size of parts of the exoskeleton during this period.