A seal"s skeletal system has the characteristics of that of a mammal. However, because they live under water, seals have evolved in such a way that they can be classed apart. The illustration shows that a seal has front flippers, back flippers and joints, which are largely covered in a thick layer of blubber. Only its "hands" and "feet" are situated outside the blubber. Different species of seal have lots of typical  differences. An example is their diets. The crabeater, for instance, lives off krill. This species has made all possible adjustments in its physique, especially in its teeth, to enable it to eat this specific type of food. Other seals have not evolved in this way.