SEM of the development of P. lamarckii. Scale bars: A-F = 10 μm; G-I = 20 μm. Apical is to the top of the figure in all panels. A. Blastula stage. B. Early gastrula, posterior view. The blastopore (b) is conspicuous at this stage. C. Posterior view of embryo showing closure of the lateral edges of the slit-like blastopore lip, resulting in formation of mouth and anus. Prototrochal (pt) cilia are visible at this stage. D. Early trochophore, right lateral view. Feeding cilia are visible around the mouth (m), which has reached its final location. The anus is located on the dorsal side of the animal (not shown). E. Complete trochophore, right lateral view. The apical tuft (at) is visible. The metatroch (mt) and neurotroch (nt) have formed. F. Late trochophore, right lateral view. Note elongation of hyposphere (hy) and general increase in size. G. Ventral posterior view of a metatrochophore larva with three segments, each marked by paired chaetae (ch). The neurotroch no longer reaches the mouth. H. Metamorphosing larvae, lateral view. The primary filaments (pf) of the branchial crown have begun to form, and loss of prototroch and metatroch has begun. The rudiments of the thoracic membrane (tm) are visible. I. Metamorphosed worm, dorsal view. The primary filaments of the branchial crown are prominent and the rudiments of the thoracic membrane and collar (co) are visible. The cilia of the newly formed faecal groove (cfg) are visible in a row along the dorsal midline. The anus (a) is now situated more terminally.