Most biologists classify organisms into five kingdoms: monera, protists, fungi, plants, and animals. It is not always obvious to which kingdom an organism belongs. For example, some protists (such as slime molds) can move from place to place under their own power—a trait normally associated with animals. However, not all such protists are multicelled (composed of more than one cell), whereas all animals are. Another example is the sponge, which is immobile as an adult and may seem to be a plant. However, biologists place the sponge in the animal kingdom because it is mobile while immature and has a cell structure like an animal. In deciding whether an organism is an animal, biologists consider a number of characteristics, including the following:

• All animals are multicelled, distinguishing them from monera and most protists.
• The cells of animals lack a cell wall, a rigid structure that encloses plant and fungus cells. Animal cells are enclosed in a pliable structure, the cell membrane.
• Most animals can move about under their own power throughout life, and all animals are mobile during at least one phase of their life. Plants and fungi are incapable of moving from one spot to another under their own power.
• Animals obtain food by eating other organisms. Plants manufacture their food from inorganic substances by a process called photosynthesis. Fungi absorb nutrients from decaying organic matter.
• Animals have a greater degree of irritability, or responsiveness, than do other organisms. Most kinds of animals have nervous systems, which enable them to respond rapidly to stimulation.