Introduction: One the first things scientists examine when classifying a new animal is the shape of its body. They look at how the external view of the animal can be divided. They ask questions like, "Does it have segments?" and "Into how many pieces can this critter be divided?"

This trait is called symmetry and it can be defined into several different categories.

The first type of symmetry is called asymmetry. This means not having any symmetry at all. Sponges are a great example of this (see image left). Sponges do not have a central axis, nor do they really have a top or a bottom or a left or right. It would be very difficult to cut up a sponge into equal pieces.

The second kind of symmetry is called radial symmetry. Organisms with this type have a central axis, usually delineated by having a top and a bottom (or dorsal and ventral) sides. If an imaginary axis were placed through the center of the animal, it could be divided into equal sections, like cutting a pizza. The jellyfish shown in the image has radial symmetry. In order for this to work, it is necessary to ignore the tentacles on the ventral side. The upper portion, or bell, of the animal is what scientists use to determine its body plan.

A more specialized type of radial symmetry is called pentamerous radial symmetry and is characteristic of the group of animals called the echinoderms. As the prefix "penta" means, this group can be divided into five equal parts about a central axis. Starfish, sea cucumbers, sand dollars, and sea urchins all have this kind of body plan. Like with the jellyfish, imagine a pin going down directly through the center of the animal. With these organisms, it is very easy to see how they can be divided into five equal parts. Note that it does not matter how many arms an echinoderm has. The basic 5-part body plan is present no matter what (and it looks like a starfish has been imprinted on the organism).

The most common type of body plan, at least among the animals, is called bilateral symmetry. This means that the animal can be divided externally into two identical halves. It is important to remember the external part here because many animals only have one of certain internal organs (stomach, liver, etc.) that are not easily divided. The best way to determine if an animal has bilateral symmetry is to see if it has two eyes. If so, then it can usually be cut directly down the middle creating left and right halves. Humans, dogs, sharks, and grasshoppers are examples of bilateral animals.

Directions for this Lesson: Answer the practice questions and then watch the videos to learn more about how scientists use body plans to classify animals.