You’ll notice two prominent, bright muscular bands along the sides of the larva. Many invertebrates have longitudinal and circumferential muscular bands that antagonize each other and control the shape and size of the animal; however, Nephtys lacks circumferential muscular bands (Clark and Clark 1960). Lateral to the bright longitudinal muscle band and running perpendicular to it are smaller muscles of the parapodia. These muscles control the lateral paddle-like projections of the polychaete body.
As a larva grows, its size (volume) will increase at a greater rate than its surface area, so the efficiency of locomotion by ciliary action alone decreases (Chia et al. 1984). My larva was about 700 µm long. The development of musculature can improve larval locomotion and extend pelagic larval residence times (Chia et al. 1984).
Chia F-S, Buckland-Nicks J, and Young CM. 1984. Locomotion of marine invertebrate larvae: a review. Can J Zool 62: 1205-1222.
Clark RB and Clark ME. 1960. The ligamentary system and segmental musculature of Nephtys. Q J Microsc Sci 101(2): 149-176.