gogol

A typical organizational plan that integrates functions and their underlying structures is bilateral symmetry and cephalization. Bilateral symmetry refers to a body that can be divided through only one plane into mirror halves. Cephalization refers to the tendency to collect specific structures toward one end of an animal body, which structures, collectively, are called a head. These include various sense organs--eyes, ears, nose, or another olfactory organ--and the mouth. Let us now look at how both bilateral symmetry and cephalization are adapted to the life a consumer, and especially, a predator, leads.

A head is selectively advantageous for an organism that first searches out its food and then eats it. In most animals, it is that part of the body that first encounters the environment; thus, the head contains sense organs to inform the body where it is going and where the food is. Such organs are coordinated with the organs of locomotion through the nervous system. And obviously, for the above reasons, it will also include the organ of ingestion, the mouth and its associated parts. A head is adaptive over the range of habits shown by consumers, from herbivores to active carnivorous predators.

Bilateral symmetry makes adaptive good sense in much the same way. A freely moving consumer that knows up from down and right from left has an advantage in being oriented to its environment. The location of prey, orientation towards it, and controlled approach or pursuit is enhanced by bilateral symmetry. Appendages are seen as a further refinement of such a body. They add to the maneuverability and the feeding capabilities of the animal.