For humans, and other animals which stand on two feet (bipeds), terms used are different from animals which stand on four (quadrupeds). Humans are described in the standard anatomical position, which is standing up with arms outstretched. So, what is on "top" of a human is the head, whereas the "top" of a dog may be its back, and the "top" of a flounder could refer to either its left or its right side. Unique terms are used to describe animals without a backbone (invertebrates), because of their wide variety of shapes and symmetry.