Below is a diagram of the external anatomy of a typical seal.
The Seal has short, thick fur, grows to be up to 6.5 feet (2 metres) long and can weigh up to 375 pounds (170 kilograms). The whiskers (called vibrissae) help the seals sense of touch. The nostrils are closed in the resting state.
Blubber is a thick layer of vascular fat found under the skin of all cetaceans, pinnipeds and sirenians. It covers the whole body, except for the appendages, loosely attached to the musculature. It can comprise up to 50% of the body mass of some marine mammals during some points in their lives.
Blubber serves several different functions, it is the primary area of fat on marine mammals, and essential for storing energy.
Seals have shorter limbs than most other mammals. Their limbs have evolved into flippers with true seals having more developed hind flippers and eared seals having more developed fore flippers.
A seals eyes are well adapted for seeing both above and below the water. When diving the seal has a clear membrane that covers and protects its eyes. In addition, its nostrils close automatically. Testicles and mammary glands are located in slits under the skin to keep the seals streamlined shape. Seals also have whiskers to help navigate and sensors in their skull to absorb sounds underwater and transmit them to the cochlea (the auditory portion of the inner ear).
A seals fingers and toes are bound together by a web of skin. Seals also have claws that are found either on the front flippers (earless seals) or back flippers (eared seals). Because water has a much higher density than air, their flippers can be much smaller proportionately in relation to their size. Additionally, seals are essentially weightless in the water, allowing them to come to a standstill and perform aquabatics in the water that would be impossible for atmospheric flying creatures.
Seals can conserve oxygen for long period of time underwater. When the seal starts diving its heart rate slows to about one-tenth of the normal rate.
The arteries squeeze shut and the sense organs and nervous system are the only organs to continue to receive a normal flow of blood. Seals are able to resist more pain and fatigue caused by lactic acid accumulation than other mammals. However, once they return to the water surface, they need time to recover and bring their body chemistry back to normal.
Eared seals, also called 'walking seals' and 'otariids', include the animals commonly known as sea lions and fur seals. These are vocal, social animals that are somewhat better adapted to terrestrial habitats with rear flippers that can turn forward such that they can move on all fours on land. Their fore flippers are larger than those of earless seals and are used as a primary source of manoeuvrability in the water. Eared seals have external ears, as their name suggests, and more dog-like snouts, further distinguishing them from the true seals.
Earless seals, also called 'true seals' or 'phocids' are the most diverse and widespread of the pinnipeds. They lack external ears and more streamlined snouts and are generally more aquatically adapted. Earless seals swim with efficient undulating whole body movements using their more developed rear flippers. The efficiency of their swimming and an array of other physiological adaptations make them better built for deep and long diving and long distance migrations. Earless seals are, however, very cumbersome on land, moving by wriggling their front flippers and abdominal muscles. True seals generally communicate by slapping the water and grunting, rather than vocalizing.