ADAPTATIONS FOR SENSING
THE ENVIRONMENT

Most kinds of animals have special body parts that respond to changes in the animal's environment. Such a stimulus (change) might come from an odor, a sight, a sound, a taste, or a touch. The simplest kinds of animals, such as sponges, have no special body parts and react to stimuli with their body cells. Animals with more complex physical structures, especially vertebrates, have highly developed organs for reacting to stimuli.

Some simple animals, such as hydras, react to stimuli with special cells. These sensory cells are scattered among the outermost cells of the body. The reactions of most other kinds of animals depend largely on one or more of the major senses. These senses are sight, hearing, smell, taste, and touch.

Some senses are more important to one kind of animal than to another. Most birds cannot find food if they cannot see it. Hearing is vital to bats. If the ears of a bat are covered, the animal will crash into objects when it tries to fly. A keen sense of smell enables dogs and wolves to find food, follow trails, and recognize danger. Taste is highly important to many insects. The butterfly finds its food by sensing the sweetness of flowers with its feet. A cat's long whiskers serve as touch organs. They enable the cat to feel its way through underbrush and avoid bumping into objects.