Sharks have inner ears, which are identifiable by the presence of two holes on either side of the shark's head. Three semicircular canals compose a shark's ear and provide a sense of balance. Each canal contains four sensory organs, called maculae, that assist in hearing.
Sharks are cartilaginous fish most closely related to skates and rays. Found in all oceans, sharks are one of the most successful animals on Earth and have existed for over 400 million years. While most bony fish have scales, a shark's skin is layered with rough, tooth-like projections called denticles that reduce drag. Sharks lack swim bladders and must keep swimming to breathe although some species, such as the nurse shark, have adaptations allowing them to remain stationary.