Dolphins are members of the Odontoceti, a suborder of cetaceans - that is, all whales, dolphins and porpoises - that means "toothed whales." There are approximately 40 species of dolphins and all are carnivores, primarily subsisting on fish and cephalopods.
In general, dolphins have a streamlined, spindle-like shape referred to by scientists as "fusiform." The dorsal fin, like the vertical tail on aircraft, is used for stability. The pectoral fins and tail flukes are used for maneuvering, akin to an aircraft's ailerons and elevator, respectively. The tail assembly serves as the sole means of propulsion.
There is only one blowhole on a dolphin, located on the dorsal side of the animal, forward of the dorsal fin. In baleen whales, such as the blue whale, humpback whale and bowhead whale, there are two adjoining blowholes.
Inside the dolphin's head is a large structure called a melon, which the animal uses to produce sonar waves for echolocation. Inside the dolphin's curved mouth are sets of conical teeth so precisely spaced that scientists have suggested that the teeth may be used as antennae for echolocation.
There are four species of river dolphins, named after the waterways in which they are found. This includes the Amazon river dolphin, the Ganges river dolphin, the Yangtze river dolphin and the Indus river dolphin.