什么是 Symmetry and cephalisation？

An animal has radial symmetry when its body is organised around a central axis with no left and right, like a hydra or a jellyfish: it meets its environment equally from all sides, and is usually fixed or drifting. It has bilateral symmetry when one plane divides it into mirror halves, defining a front (anterior) and rear (posterior), a back (dorsal) and a belly (ventral): the plan of an animal that moves head first, with sense organs and nervous centres concentrated at the front (cephalisation).