WHALES
Whales are the largest members of a group of mammals known as cetaceans, from the Greek word for "Whale." The smaller members of the same group are called dolphins or porpoises. The difference between a whale and a dolphin or porpoise is almost entirely one of size.
All cetaceans are warm-blooded, air-breathing mammals that have become adapted to life in the water. They are no more closely related to fishes than you are. Their ancestors lived on dry land and went about on four legs.
Scientists divide present-day cetaceans into two groups: Odontoceti and Mysticeti. Odontoceti means "whales with teeth." The whales in this group have teeth. Mysticeti means "moustached whales." These whales have "moustaches" of baleen, or whalebone, hanging from the roof of the mouth. Baleen is a fibrous, horny substance, fringed along the inner edges. Using their baleen, these whales strain huge quantities of small food out of the water.
The Odontocetes, or Toothed whales
Most odontocetes belong to the family known as Delphinidae, or delphinids. And most members of this family are dolphins. A few reach such large size that they are called whales.
The two best known of the giant delphinids are the pilot whales (also called blackfish and potheads) and the killer whales.
The Mysticetes, or Baleen whales
When people speak of whales, they usually mean the mysticetes, or baleen whales. The largest animals ever to live on earth belong to this group. One kind, the blue whale, may be 100 feet long and weigh more than 120 tons, although no blue whale of this size has been seen for many years. Even the biggest dinosaurs did not reach this size. Most baleen whales are truly gigantic, though the smallest baleen species, the pygmy right whale, is less than 20 feet long.