Animals are multicellular heterotrophs whose cells lack cell walls. At some point during their lives, all animals are capable of movement, although not all animals have muscles they use for this. In the most commonly encountered animals, the mobile stage is the adult, although some animals (such as corals and sponges) have sessile (or nonmobile) adult phases and mobile juvenile forms. Both animal and plant evolutionary history show the development of multicellularity and the move from water to land (as well as a secondary adaptation back to water, for example dolphins, whales, duckweed, and elodea).
Animals developed external or internal skeletons to provide support, skin to prevent or lessen water loss, muscles that allowed them to move in search of food, brains and nervous systems for integration of stimuli, and internal digestive systems.
