Body Symmetry and Cephalization in Animals

0:00Animals range from simpler to more complex. One clear trend is in the symmetry of their bodies.

0:07Start with a sponge, one of the simplest animals.

0:11A plane of symmetry is an imaginary cut that splits a body into two mirror-image halves.

0:18Most sponges have no such plane: wherever you cut, the halves differ. They are asymmetrical, meaning without symmetry.

0:27Next, a jellyfish, seen here from above. Its parts are arranged in a circle around a centre.

0:35Many planes through the centre split it into matching halves. This is called radial symmetry.

0:42With no front or back end, it meets its surroundings equally from all sides.

0:47Most animals are built like this flatworm, seen here from above.

0:52Only one plane, from head to tail, splits it into left and right mirror halves.

0:58This is called bilateral symmetry, which means two sided.

1:03It has a front end, called anterior, and a back end, called posterior.

1:10It moves front end first, so sense organs, like these eyespots, are gathered there.

1:16Nerve cells gather there too, forming a simple brain linked to nerve cords along the body.

1:23Gathering nerves and sense organs into a head at the front end is called cephalization.

1:30So the trend runs from asymmetrical, to radial symmetry, to bilateral symmetry with a head.

1:37In short, most animals are bilateral, with nerves and sense organs gathered in a head.