There are three formal periods of early trilobite development:
Protaspid: larval form comprised of the beginnings of indistinguishable cephalon and pygidium. Then, a transverse furrow appeared between the cephalon and pygidium (i.e., head and tail) from which two new thoracic segments grew.
Meraspid: New segments appear posterior to the pygidium (pygidial segments) as well as anterior to the pygidium (thoracic), with those anterior to the pygidium becoming the thoracic segments. All new segment articulated and could thus freely flex.
Holaspid: Starts after full complement of thoracic segments of a mature trilobite had developed in the Meraspid. Ecdysis continued during the holaspid stage of development, though with no additional segments added to the thorax.

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