Another type of mammal is the marsupial. A type of marsupial is the kangaroo. The marsupials are special because they develop internally, but there are not enough nutrients in the egg sac to nourish the baby for the full amount of time. So, the baby is born early and it goes into a pouch on the mother's belly and it stays there until it is old enough and totally developed. The mother's pouch protects and feeds the baby at the same time. The last kinds of mammal are the placental. Placentals have a placenta, which allows the baby to develop inside the mother for a longer period of time. This gives the baby a lot of parental care. All the babies" nutrients are exchanged through the mother. Internal development of a child is a more efficient way to protect young until they are developed enough to be born. The longer they are in the womb the more likely of a chance they have to survive. This is why mammal reproductive strategies are efficient for their environments. Reptiles have internal fertilization. Once the eggs are fertilized, different reptiles do different things to their young. Turtles come ashore to bury their eggs and they leave their young with no parental care. Some take minimal care of their eggs. Snakes warm themselves up and then incubate their eggs, but once the eggs hatch, the young are on their own. Some crocodiles and alligators stay with their parents for up to two years. Reptiles have a special type of egg. It has a leathery shell and several membranes to protect the embryo. It also has an amnion, which specially encloses the embryo and a yolk to give it nutrients. Because of these special eggs, it allows reptiles to live their entire life out of water. Most fish lay eggs. Some have external fertilization and some have internal fertilization. Fish may lay hundreds or millions of eggs. Some protect their eggs in their mouths or hide their eggs to protect them.