Bilateral Symmetry

Bilateral symmetry is an arrangement of body parts such that an organism can be divided equally by one cut passing through it longitudinally. An organism that is bilaterally symmetrical has mirror image right and left sides. An animal with bilateral symmetry has a distinct head, or anterior end; a tail, or posterior end; a back, or dorsal, surface; and a bottom, or ventral, surface. The brain, mouth, and sense organs are usually located in the head.  Most bilaterally symmetrical animals are active and travel headfirst through their environment, their eyes and other sense organs contacting the environment first. The photo shown here is of the left side of Conure bird, which belongs to the phylum Chordata (which humans also belong to). Chordates are deuterostomes, a branch of bilaterians, which are bilaterally symmetrical animals. The Conure can be cut in half longitudinally and have mirror right and left sides.


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