What is the cause behind the nature, choosing forward directional locomotion to develop in the metazoans (as in bilaterians which is the main cause to cephalisation) over multi-directional movement (a much higher form) which already existed in the radially symmetrical jelly-fishes even before the bilaterians appeared ?
There are advantages to both types of body symmetry. a bilaterian fish enjoys much faster, unidirectional movement compared to a jellyfish. This could be advantages if the bilaterian is a predator or a prey for example. in the cambrain explosion it is believed that predator-prey coevolution drove much of the adaptive radiation that resulted in the emergence of a large number of species. Predators which can move faster are able to more easily catch prey and prey which are faster can easily escape.
A second advantage to a bilaterian body plan other than the ability to be more active is that the sense organs and nervous system can be situated anteriorly. This enables the organism to react more quickly to stimuli and as the head reason will be the first to experience a new environment (think about a worm sticking its head out of the soil)the organis is able to react more quickly.
Radially symmetrical bodies are much slower although they tend to be better for sessile organisms which dont have to move such as sponges, corals, echinoderms etc.