The order Cetacea includes the marine mammals known as dolphins, whales, and porpoises. The sub-orders include; Mysticeti, Odontoceti, and Archaeoceti. The Archaeoceti are ancient, extinct whales. Cetus is Latin and means "whale". its original meaning, "large sea animal", was too general. Some Cetacean species are highly intelligent and scientific movements have been made to classify them as "non-human persons." Cetology is the branch of marine science that deals with the study of cetaceans. Cetaceans are the mammals best suited and adapted to aquatic life. Their bodies are fusiform (spindle-shaped). They have forelimbs modified into flippers and are nearly hairless. The tiny hind limbs are vestigial; they do not attach to the backbone and are hidden within the body. Cetacean's are insulated with a thick layer of blubber to keep warm in the cool water they live in. Cetaceans breathe air. They surface periodically to exhale carbon dioxide and inhale a fresh supply of oxygen. During diving, The blowholes are closed by a muscular action. When the Cetacean surfaces the blowhole opens and warm air is exhaled. Cetacean's blowhole's have evolved to a position on the top of their heads to simplify breathing. Cetacean's can hold their breath much longer than most mammals, which is approximately 7-30 minutes. It varies by species. The higher the myoglobin concentration in skeletal muscle, the longer the animal can stay underwater. Cetacean's have small eyes set on the side instead of the front of the head. This means that only cetaceans with pointed 'beaks' (such as dolphins) have good binocular vision forward and downward. The lens is almost spherical, and is most efficient at focusing the minimal light that reaches the deep water. They have tear glands that secrete greasy tears to protect the eyes from salt water. Cetacean's make up for their poor vision (with the exception of the dolphin) with excellent hearing. As with the eyes, Cetacean ears are also small. Sea life is the evolutionary cause of the loss of external ears. With external ears, the purpose is to collect and focus airborne sound waves. However, water is a better sound conductor than air, so the external ear is not necessary. The ear is a tiny hole in the skin just behind the eye. It is so highly developed it can detect sounds from dozens of miles away and determine where the sounds come from. The sub-order Odontoceti are capable of echolocation. Echolocation occurs when the animal sends out a series of high frequency clicks which in turn bounce off of the objects surrounding them, causing an echo from the external stimuli. From these echoes the animal can determine the size and shape of the objects around them. This echolocation is so advanced in most Odontoceti that they can distinguish from prey and non-prey. At least one species of Cetacean, the Tucuxi is capable of electroreception to sense prey. This involves electric signals being sent to the Tucuxi's electricity sensing organs to develop a sense of the environment. Cetacean's have teeth that they use to catch prey such as fish, squid, and seal but they do not chew their catch. Instead they swallow them whole. If the prey is too big to swallow whole they will eat them chunks at a time. The Blue whale and Sperm whale often eat whole schools of plankton at a time. Cetacean's are incredibly smart animals. Research has shown that dolphins understand generally human concepts such as numerical continuity ( but not necessarily counting). Scientists usually classify dolphin intelligence to the equivalence of an elephant. Dolphins are some of the only animals that are capable of self-recognition. Not only can they recognize themselves, but they can also watch TV. Scientist Lou Herman exposed dolphins to a TV screen and immediately saw resulting actions. Dolphins also use tools to solve problems. Scientists have observed a dolphin coaxing reluctant moray eels out of its crevice by killing a scorpion