The way a manatee moves is because of his internal skeleton. A manatee's flippers have finger-like bone structures similar to a human hand which enables them to move through the water, bring food to their mouths and even hold objects. On their flippers they have three or four nails which give them protection and a better grip on food and other objects. Manatees also have a pelvic bone that is not attached to any other bone in their body. This pelvic bone is remains from when Manatees once lived and walked on land. The manatees ribs and other long bones lack marrow cavities which gives them a heavier skeleton.
Manatees have a very low amount of myglobin which is hemoglobin in the muscles which is very unusual in diving animals which means they can't hold their breathe for longer than 15 to 18 minutes despite their large oxygen exchange and shallow dives.
The manatee has a closed circulatory system with a four chambered heart.