I don’t know what the exact number is but amphibians (e.g. frogs, newts) are animals that can breathe both in and out of water…and there are at least 6000 species!
Crayfish can breathe out of water, but only for a limited amount of time (they have to return to water eventually or they’ll die). So can the mudskipper, an amphibious fish species (check out this video here: http://www.youtube.com/watch?v=KurTiX4FDuQ )
1. lungs – like we do (and most land animals and things like whales, seals and dolphins as well)
2. gills – like fish and early stage amphibians (tadpoles)
3. through their skin
4. through their trachea (this are special tubes that an insect has for breathing).
Many animals live in water but don’t actually breathe underwater – for example all mammals that live in the sea have to come to the surface to breathe. A whale or dolphin would drown if it stayed underwater for too long.
Fish breath underwater through gills – these are a special organ that absorbs oxygen from the water. The gills have a large surface area to allow as much oxygen to pass from the water into the fish. Again, if a fishes gills get blocked then it will suffocate.
Amphibians like frogs, toads and newts are very interesting. When they are young tadpoles they have gills and breathe like fishes and live in water. However as they get older and turn into adults they lose their gills and develop lungs so that they can breathe on land. However, frogs can also absorb oxygen through their skin – so they can hibernate at the bottom of a pond all winter for example.
One of my favourite amphibians is the axolotl – this amphibian doesn’t lose it’s gills as it gets older and stays in its ‘tadpole’ form all its life, living in water. They also look really funny (do a google image search for axolotl to see!) http://en.wikipedia.org/wiki/Axolotl