An animal is a multicellular organism that eats other organisms for energy, lacks rigid cell walls, and passes through a hollow-ball embryonic stage called a blastula during development. Those three traits, taken together, separate animals from every other form of life on Earth. But the full picture involves several more defining features, and some surprising exceptions that test the boundaries of the definition.
Cells Without Walls
Every animal cell is a eukaryotic cell, meaning it has a nucleus enclosed in a membrane. That much is shared with plants and fungi. What sets animal cells apart is what they’re missing: a rigid cell wall. Plants have cell walls made of cellulose; fungi have walls made of chitin. Animal cells have only a flexible plasma membrane holding everything together.
This sounds like a disadvantage, but it’s the opposite. The lack of a rigid wall allowed animals to develop a far greater diversity of cell types, tissues, and organs than plants or fungi ever did. Muscle cells that contract, nerve cells that fire electrical signals, red blood cells that squeeze through capillaries: none of these would work if they were locked inside stiff boxes. The flexibility of animal cells is what makes complex body plans possible.
To compensate for having no cell wall, animal bodies rely on a protein scaffold called the extracellular matrix. The key structural protein in this matrix is collagen, which forms fibers that resist pulling forces and give tissues their strength. Elastin fibers add springiness, while a gel-like substance of sugar-protein molecules resists compression and lets nutrients flow between blood and cells. This system does the structural work that cell walls do in plants, just in a completely different way.
Eating by Ingestion, Not Absorption
Animals, plants, and fungi all need organic molecules for energy. Plants make their own through photosynthesis. Fungi and animals both consume other organisms, which makes them heterotrophs. But the method is fundamentally different.
Fungi digest first, then absorb. They secrete enzymes outside their bodies to break down food externally, then soak up the nutrients through their cell walls. Animals do the reverse: they ingest first, then digest. Food enters the body (through a mouth, or by engulfing particles at the cellular level), and digestion happens internally. This distinction holds across the entire animal kingdom, from a sea sponge filtering bacteria out of water to a lion tearing into a carcass.
The Blastula: An Embryonic Signature
One of the most reliable ways to identify something as an animal is to look at how it develops. Almost all animals pass through a specific sequence of embryonic stages that no other kingdom shares.
After fertilization, the single-celled zygote undergoes a rapid series of divisions called cleavage. These divisions are unusually fast because the cells aren’t growing between splits; the large zygote is simply being carved into smaller and smaller cells called blastomeres. By the end of cleavage, those cells have arranged themselves into a hollow sphere: the blastula. This hollow ball of cells is a defining feature of animal development.
Next comes gastrulation, where the blastula folds inward on itself to create a two-layered structure with an opening to the outside. This stage produces the three foundational tissue layers (ectoderm, endoderm, and mesoderm) that go on to form every organ in the adult body. The outer layer becomes skin and nerves. The inner layer becomes the gut lining. The middle layer becomes muscle, bone, and blood. Variations on this process exist across the animal kingdom, but the basic pattern is remarkably consistent from jellyfish to humans.
Nervous Systems and Movement
Most animals can move, and most have some form of nervous system. These two traits are deeply connected. Nerve cells and muscle cells working together allow animals to sense their environment and respond quickly, something plants and fungi simply cannot do.
Nervous systems are considered a hallmark of animal evolution. Even very simple animals like jellyfish have nerve nets that coordinate swimming and feeding. More complex animals evolved centralized brains, specialized sensory organs, and increasingly sophisticated behaviors. The ability to detect light, chemicals, pressure, or sound and then act on that information in real time is one of the most distinctive things about being an animal.
Movement doesn’t have to mean running or swimming for an organism’s entire life. Corals, for example, are sessile as adults: they anchor to rocks and never move again. But their larval stage, called a planula, swims freely through the water. Sea anemones can creep slowly along surfaces. The general rule is that animals are motile at some point in their life cycle, even if the adult form stays put. Cnidarian muscles perform functions in locomotion, defense, feeding, and digestion across all life stages.
Body Plans and Genetic Control
Animals with symmetrical bodies (bilateral symmetry, like humans, or radial symmetry, like starfish) share a set of master control genes called Hox genes. These genes act as transcriptional regulators: they tell cells where they are along the head-to-tail axis of the body and what structures to build there. Hox genes control axial patterning in all bilaterally symmetrical animals, from insects to fish to primates.
Most animals are also diploid, carrying two copies of their genetic material in every cell. And most reproduce sexually, producing specialized egg and sperm cells that fuse during fertilization. Asexual reproduction does occur in some species (certain starfish can regenerate from fragments, and some insects reproduce without mating), but sexual reproduction is the dominant pattern across the kingdom.
Sponges: Animals Without Tissues
Sponges are the test case that forces biologists to think carefully about what “animal” really means. They have no nervous system, no muscles, no organs, and no true tissues. They can’t move as adults. They look more like lumpy rocks than anything you’d call an animal.
Yet sponges are unambiguously classified as animals. They are multicellular heterotrophs that lack cell walls, pass through a blastula-like stage in development, and ingest food rather than absorbing it. Their cells include a specialized type called a choanocyte, or collar cell, which has a tiny funnel-shaped collar that traps food particles. These choanocytes bear a striking resemblance to choanoflagellates, a group of single-celled organisms considered the closest living relatives of all animals. That structural similarity suggests animals may have evolved from choanoflagellate-like ancestors hundreds of millions of years ago.
Sponges show that the definition of “animal” doesn’t require complexity. It doesn’t require a brain, a heart, or even organized tissues. What it requires is a specific combination of cellular, developmental, and nutritional traits: eukaryotic cells without walls, a blastula stage, multicellularity, and heterotrophy through ingestion. Sponges meet every one of those criteria, even though they diverged from all other animal groups very early in evolutionary history.
How Animals Differ From Other Kingdoms
- Animals vs. plants: Plants make their own food through photosynthesis, have rigid cellulose cell walls, and generally cannot move. Animals eat other organisms, lack cell walls, and are motile at some life stage.
- Animals vs. fungi: Both are heterotrophs, but fungi digest food externally and absorb nutrients through their cell walls. Animals take food inside their bodies and digest it internally. Fungi have cell walls made of chitin; animals have none.
- Animals vs. protists: Some protists are single-celled and heterotrophic, which can make them look animal-like under a microscope. But animals are always multicellular, and their embryonic development through a blastula stage is unique to the animal kingdom.
No single trait on its own makes something an animal. Plenty of non-animals are multicellular, heterotrophic, or motile. It’s the full package, flexible cells without walls, internal digestion, blastula development, and (in most cases) a nervous system and the capacity for movement, that defines the roughly 1.5 million known species in the animal kingdom.