Whales have no hind limbs and their forelimbs have become modified into broad flippers. The tail flukes are horizontal in all the cetaceans (unlike fish tails which are vertical) and the movement of this powerful tail propels them through the water. Some whales have a dorsal (top) fin, and certainly most of the dolphins and porpoises have them. The nostril or blow hole is situated on top of the head and is automatically closed like a watertight valve when the animal submerges. A layer of blubber covers the body beneath the skin and acts as insulation against the extreme cold by conserving body heat.
The whale family is divided into two main groups: the baleen whales which have bony strips or plates in their mouths instead of teeth; and the toothed whales which have teeth. Of the 90 or so species of Cetaceans, about 80 are toothed whales, including 50 species of dolphins and porpoises. There is very little difference between dolphins and porpoises and they are closely related to each other, but it is possible to establish which is which by the shape of the head and mouth. The dolphin has a beak-like snout whereas the porpoise is usually smaller and slimmer and has a rounded face and a flat mouth.
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