Cross references:    Myomere    Amphioxus Muscles  
Amphioxus Locomotion  

Myoseptum definition of Myoseptum in the Free Online Encyclopedia. 
a wall of connective tissue that separates the myomeres in lancelets, vertebrate animals, and humans. The myosepta stretch between the axial skeleton and the skin, thus supporting the muscle fibers of the myomeres. (Fish have muscle ossicles in the myosepta.) Fish and amphibians also have a horizontal myoseptum that divides the myomeres into dorsal and ventral sections. In tailless amphibians and in the Amniota (with the exception of snakes and lizards), myosepta are present only in the embryonic stage. In man, the vestiges of myosepta are the connective-tissue walls of the rectus abdominis."

Swimming in the Cambrian » Pharyngula (Goog) 
    Good diagrams. 
The body musculature of fish is most elegantly organized into repeating blocks of muscle along the length of the animal, each segment having a chevron (“V”) or “W” shape. Here’s a pretty stained photo of a 30 hour old zebrafish to show what I mean; it’s a little weird because this one is from an animal with experimentally messed up gene expression, all that red and green stuff, but look at the lovely blue muscle fibers stretching across the length of each segment. 

Lateral view of a 30 hour old zebrafish, embryonic myotome, displaying ectopic eng2a:eGFP (green) fibers in response to Smad7 (marked in red) expressing clones. Slow muscle fibers are marked by monoclonal antibody F59 (blue)."  
    "Each of those muscle fibers is attaching to a connective tissue band in the line between each block of muscle, called a myoseptum. The force of contraction of the muscles is going to pull on these connective tissue anchor points, stressing them. By organizing all of the fibers to pull at an angle rather than perpendicularly to the myoseptum, it reduces the amount of force directly applied to the septal connections by as much as 50-60%, depending on the angle, relative to the overall force of contraction applied to generate the bending movements.

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.

An additional benefit is that the angle distributes the force generated by a single segment’s contraction to a longer stretch of the notochord, the central springy rod that forms the main axis of the skeleton.

So why do fish do this? Because with powerful muscle contractions needed for fast swimming comes the potential for tearing connective tissue and buckling or damaging the central skeleton. These are adaptations for rapid, efficient swimming. They are also necessary for escape responses: startle a fish, and it makes an abrupt, massive contraction of these muscles to flick it out of danger. That behavior wouldn’t be so useful if it ended up tearing muscle insertions and crippling the fish within seconds of its escape."