Brains. They think with their feet.
Although the above answer is sometimes true. Horses, Pigeons and yes Rats, actually lack a Gall Bladder. They do not have one, interesting, eh?
horses
lungs
Yes, donkeys do have a gallbladder. The gallbladder is an organ that stores bile produced by the liver, which aids in the digestion of fats. While some animals, like horses, lack a gallbladder, donkeys possess this organ as part of their digestive system.
What organ systems do echinodermes lacking
Neither rats nor horses have the ability to regurgitate (throw up). As gross as it sounds, the ability to throw up something bad is a life-saver.
Horses, Bats, and Rats are all mammals, give birth to live young, and nurse their live young.
If you are talking about other versions, rats or white mice were turned into the horses. The original story was slightly different.
aplasia
Yes - only rats and horses do not have a gall bladder.
Deer do not have a gallbladder. Scientists have still not been able to establish why they lack this organ that's responsible for producing bile that aids in digestion.
Rats do not have a gallbladder because their liver produces bile continuously, so they do not need a storage organ like the gallbladder to store bile.
Humans have a gallbladder, which rats do not possess. The gallbladder stores bile produced by the liver and helps in the digestion of fats. Rats do not have a gallbladder and instead secrete bile directly from the liver into the small intestine.