Marine invertebrate larvae swim by using cilia or muscles, or a combination of these. The effectiveness of cilia as locomotory organelles diminishes with increasing body size above 1 mm. Thus, larvae propelled by cilia are small and, owing to the small Reynold's numbers that operate in this size range, their movements are governed by viscous forces rather than inertial ones. Cilia may be distributed uniformly over the surface of the larva and (or) localized on rings, bands, arms, or lobes. During development the pattern of ciliation may change; this often increases the swimming ability of the larva, particularly its manoeuverability. In many cases, redistribution of cilia coincides with the onset of feeding behavior. The locomotory currents produced by ciliary beating or the action of swimming appendages may simultaneously convey food particles to the mouth. Muscles may have enabled some larvae to exceed the size limit imposed by ciliary propulsion and also have enabled greater swimming speeds. Invertebrate larvae that use muscular locomotion possess some form of skeleton (hydrostatic, exoskeleton. or notochord) to provide the necessary resistance for muscular contraction. The density of most marine invertebrate larvae exceeds that of seawater, therefore, they must swim to stay suspended. A wide variety of parachute structures, density-reducing devices, and passive hydrodynamic mechanisms counteract the sedimenting effects of gravity. The timing of development in some larvae is such that when the tendency to sink exceeds the ability to swim, the larva is preparing for settlement and metamorphosis.
