I'm going to remove the word "designed", lest we spark some heated debate on evolution and design!
Does anyone know whether these structure break off, reabsorb, or degenerate? I'm going to take the easy way out and say disappear:
Balancer: Rod-like structures on each side of the head of newly-hatched larvae. The balancers stop the larva from sinking into sediment or losing its balance. Once the forelimbs develop, the balancers disappear.