Scanning electron miscrosopic observations of the larval development of the reef-building polychaete Phragmatopoma lapidosa

The scanning electron microscope (SEM) was used in a study of the larval development of the reef-building sabellariid polychaete Phragmatopoma lapidosa. Mature sperm are modified from the 'primitive' polychaete plan by possessing a tapered and bent acrosome. The outer surface of the irregularly shaped, freshly spawned egg envelope is granular in appearance. SEM observations of various stages in development from the early trochophore through larval metamorphosis and the early juvenile stages suggest that the egg envelope serves as the cuticle through the trochophore stage but is then replaced by another cuticle penetrated by microvilli. SEM has revealed the presence of 'sensory tufts' on the dorsal surface of the larval tentacles which may play a role in larval substrate selection.