It is believed that some fifty million years ago, these animals left the
land and went into the sea, continuing to breathe air but evolving into
an aquatic lifestyle. The manatee's hind legs have been reabsorbed and
only a cartilaginous structure remains.
The similarities between manatees and elephants are considerable: the toenails, teeth, digestive tract, prehensile mouth parts, skin, location of mammary glands, and hair are the most notable.
They have a small head with two high nostrils, modest but useful flippers, and a bulbous round body tapering to a flat round tail that propels them through the water with amazing efficiency.
They usually swim slowly and sometimes creep along the bottom with their flippers.
The manatee's eyes seem tiny for such a large animal, but it is believed they have excellent close vision, which makes sense, since manatees do spend so much time in waters that are turbid and cloudy.
It is known that manatees can hear very well, but appendages resembling ears are nowhere to be found. Their organ of hearing is, in fact, a tiny hole just behind their eyes. Like dolphins and whales, these work extremely well underwater, since sound travels six times better in water than in air.
Manatee hair is very thin and widely spaced, but the hair around their mouth is stiff and brush-like. Their mouths are also equipped with large, muscular, prehensile mouth flaps, which are useful for collecting algae and grass as they forage for food.
Although manatees are primarily vegetarians, their protein intake is supplemented by the many tiny animals that live in the grasses and algae they feed on. These hefty animals typically consume ten to twenty percent of their body weight in food every day (some 100 to 200 pounds of soggy grasses and weeds).
Manatees use their flippers in a variety of ways, but mostly for steering as they swim. They even have fingernails, but these vestigial appendages more accurately resemble the toenails of an elephant. Their teeth are also a lot like the elephant's, and add to the evidence supporting the theory that elephants and manatees have common ancestors.
Source: http://www.xtalwind.net/~cfa/2anatomy.htm