Fig. 9. The dynamics of swimming in ascidian larvae. A free-swimming larva is drawn from a dorsal and posterior perspective for an instant of time when the tail is beating towards the left. Some of the forces likely to influence the swimming mechanics are drawn to illustrate how moments acting on the center of mass could be generated in one (A) or two (B) dimensions. (A) Only hydrodynamic forces acting on the frontal plane of the body are drawn. As this larva swims forward, a yawing moment in the counterclockwise direction will tend to rotate the body in a counterclockwise direction around an axis perpendicular to the frontal plane. If only these hydrodynamic forces acted on the body for the entire tail beat, this larva would follow a circular trajectory lying on a plane coincident with the frontal plane of the body. (B) In addition to hydrodynamic forces, this larva has a buoyancy force and the weight of the body acting on it. The buoyancy force acting at the body’s center of volume is posterior to the center of mass and therefore generates a pitching moment. With both pitching and yawing moments, the body would tend to rotate around an axis that is not perpendicular to the frontal plane. As a result, the larva would swim along a helical trajectory.
