The trochoblasts in the pilidium larva break an ancient spiralian constraint to enable continuous larval growth and maximally indirect development.

The pilidium exhibits a familiar relation between identified blastomeres and the primary ciliated band, but the manner in which these cells form this organ differs fundamentally from the way equivalent cells construct the trochophore's prototroch. Also, the establishment, by some progeny of the putative trochoblasts, of indeterminate stem cell populations that give rise to juvenile rudiments, as opposed to an early cleavage arrest, implies a radical alteration in their developmental program. This transition may have been essential to the evolution of a maximally indirect developing larval form-the pilidium-among nemerteans.