Just when you think you know all the mammals...

... someone posts a photo of a yapok (water opossum).

Being a marsupial and at the same time an aquatic animal, the water opossum has evolved a way to protect its young while swimming. A strong ring of muscle makes the pouch (which opens to the rear) watertight, so the young remain dry, even when the mother is totally immersed in water. The male also has a pouch (although not as watertight as the female's), where he places his genitalia before swimming. This is thought to prevent them from becoming tangled in aquatic vegetation and is probably helpful in streamlining the animal as well.