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
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.
Also see: Powerpoint slide