A Penguin is a short-legged,flightless bird of cold southern, especially found in Antarctic regions.Most penguins feed on krill, fish, squid, and other forms of sealife caught while swimming underwater.The anatomy of a penguin is interesting. They spend half of their life on land and half in the oceans.The scientific classification of the penguin has been discussed here.

Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Order:	Sphenisciformes
Family:	Spheniscidae

Scientists recognize 32 species of extinct penguins.Penguins probably evolved from flying birds more than 40 million years ago . As the ancestors of penguins became adapted to an oceanic environment, structural changes for diving and swimming required the loss of flying adaptations.To date, the discovery of all penguin fossil fragments has been limited to the Southern Hemisphere.Scientists believe that ancient penguins began disappearing about the same time that the number of prehistoric seals and small whales started increasing in the oceans. Some scientists hypothesize that seals, whales, and penguins may have competed for the same food source, and that the penguins may have become prey themselves. Both factors may have contributed to their extinction.

The origin of the word "penguin" has been a subject of debate. Researchers' and historians' theories range from references to the amount of fat (penguigo in Spanish and pinguis in Latin) penguins possess to the claim that the word was derived from two Welsh words meaning "white head".

Penguins are superbly adapted to an aquatic life. Their wings have become flippers, useless for flight in the air. In the water, however, penguins are astonishingly agile. Within the smooth plumage a layer of air is preserved, ensuring buoyancy. The air layer also helps insulate the birds in cold waters. The plumage of penguins in tropical and temperate zones is much thinner than that of more southern species.

On land, penguins use their tails and wings to maintain balance for their upright stance.

All penguins have a white underside and a dark (mostly black) upperside. This is for camouflage. A predator looking up from below (such as an orca or a leopard seal) has difficulty distinguishing between a white penguin belly and the reflective water surface. The dark plumage on their backs camouflages them from above. Diving penguins reach 6 to 12 km/h, though there are reports of velocities of 27 km/h (which are probably realistic in the case of startled flight). The small penguins do not usually dive deep; they catch their prey near the surface in dives that normally last only one or two minutes. Larger penguins can dive deep in case of need. Dives of the large Emperor Penguin have been recorded which reach a depth of 565 m (1870 ft) and last up to 20 minutes.

Penguins have an excellent sense of hearing. Their eyes are adapted for underwater vision, and are their primary means of locating prey and avoiding predators; in air, conversely, they are nearsighted. Their sense of smell has not been researched so far. They are able to drink salt water safely because their supraorbital gland filters excess salt from the bloodstream.The salt is excreted in a concentrated fluid from the nasal passages. Penguins have no external genitalia.Consequently, chromosome testing must be done in order to determine a penguin's sex.