Morphogenesis of larval cuticle in the polychaete Phragmatopoma lapidosa. A correlated scanning and transmission electron microscopic study from egg envelope formation to larval metamorphosis.

The development of the egg envelope and its incorporation into the larval cuticle of the polychaete Phragmatopoma lapidosa, was studied by correlative scanning and transmission electron microscopy. The mature egg possesses an envelope composed of five zones including an outer granular zone formed by the tips of the egg microvilli. The formation of the granules is described and their functions are discussed. The entire egg envelope is retained as the larval cuticle up to the 16 h trochophore stage. From this stage to about the 60 h larval stage, the envelope is gradually lost and replaced by a cuticle consisting of branching microvilli. The cuticle of the 20 day larva is composed of highly branching microvilli penetrating a homogeneous electron opaque cuticle. The possible functions of the cuticle among the Annelida are discussed.