Figure 27.1 This shark uses its senses of sight, vibration—lateral-line system—and smell to hunt, but it also relies on its ability to sense the electric fields of prey, a sense not present in most land animals. (credit: modification of work by Hermanus Backpackers Hostel, South Africa)

All bilaterally symmetric animals have a sensory system, the development of which has been driven by natural selection. Thus, sensory systems differ among species according to the demands of their environments. Animals’ senses are constantly at work, making them aware of stimuli, such as light or sound, or the presence of a chemical substance in the external environment. They also monitor information about the organism’s internal environment. The shark pictured above has the ability to perceive natural electrical stimuli produced by other animals in its environment, a sense called electroreception. This enhanced ability to sense prey gives the shark an evolutionary advantage over other fish. While it is helpful to this underwater predator, electroreception is a sense not found in most land animals. You can read more about electroreception in sharks at the Sharks info website.