Figure 2

The beneficial consequences of having chevron- or V-shaped myomeres, illustrated using amphioxus. (A) The V shape guarantees that the force acting perpendicular to the myoseptum (vector P) is less than the force of contraction (vector F) by an amount that increases with increasing angle (Θ) to the vertical. The degree of incline shown is typical for amphioxus larvae, and increases with increasing age. (B) Somite overlap in young adult amphioxus, modified from [11]. The central components of the locomotory system are the notochord (not, shown in violet) and the nerve cord (green). These are bound to the myomeres (pink) by sheaths of basal lamina (blue). The V-shaped myomeres are positioned so that the tip of the caudal-most in any section is adjacent to the notochord, while the extended tails of progressively more anterior myomeres (shown in progressively lighter shades, compare with A) are ranged above and below. Because every point along the notochord has essentially the same complement of septa, this arrangement ensures that the force of contraction experienced by the notochord is distributed evenly along its anteroposterior axis, rather than being borne at specific sites