What is Bilateral Symmetry system?
To review: all animals besides the sponges (Phylum Porifera) belong to the Eumetazoa. There are two groups within the Eumetazoa that possess radial symmetry: Phylum Cnidaria and Phylum Ctenophora (the comb jellies). The rest of the Eumetazoans all have bilateral symmetry.
Bilateral symmetry refers to a body, which, if divided down the middle, would produce two opposite halves that form mirror images of each other. For instance, imagine drawing a center line down the middle of your body that extends from the top and middle of your head down between your eyes, down the middle of your nose, down through your breast bone, through your belly button, and all the way through your pelvic bone. You are now divided into two equal halves. Since each half is a nearly identical image of the other, the halves are symmetrical. Since there are two symmetrical "sides" to your body, it is referred to as a bilaterally symmetrical body.
Whereas in a radially symmetrical body there is no front or back, bilaterally symmetrical bodies do have fronts and backs. The front end is referred to as the anterior, the back end as the posterior. The top of a body is referred to as the dorsal portion of the body, and the bottom, or belly, of an organism is referred to as the ventral portion. The direction toward the sides is designated lateral, and the direction toward the centerline is called medial.
The development of a bilaterally symmetrical body plan with an anterior and posterior is thought to represent a major evolutionary advancement. Radially symmetrical organisms tend to spend their lives "fixed" in one place, whereas animals with an anterior-posterior structure can move about more efficiently. They are able to concentrate their sensory and feeding organs forward, where they function most efficiently. The importance of this ability becomes apparent when you consider animals' needs in locating and capturing food, as well as avoiding danger.