Characteristics of Animals and Body Organization
Animals are multicellular organisms that obtain food by eating other organisms, have cells without cell walls and, in most species, can move and respond quickly to their surroundings. In brief: animals belong to the kingdom Animalia, and their bodies are organized into cells, tissues, organs and organ systems. A dog, a spider and a jellyfish are all animals, even though they differ greatly in body plan, symmetry and the number of tissue layers.
More than a million animal species have been described, from sponges to whales. This guide explains the common characteristics of animals, how animal bodies are organized, the main body plans such as symmetry and body cavity, how to classify an unknown animal step by step and the mistakes that often arise.
What are the characteristics of animals?
Most biologists agree on a set of traits that animals share. A few species are exceptions to some of them, so the list works best as a whole.
- Multicellular: The body consists of many cells that cooperate.
- Eukaryotic cells without cell walls: Animal cells have a nucleus and are surrounded by a flexible membrane, not a rigid wall.
- Heterotrophic: Animals cannot make their own food and must eat other organisms, usually by ingestion and internal digestion.
- Movement: Most animals can move at some stage of life, using muscle cells. Adult sponges and corals are fixed in place, though their larvae swim.
- Nervous system and sense organs: Nearly all animals have nerve cells that detect stimuli and coordinate responses. Sponges are a main exception.
- Sexual reproduction: Most animals reproduce sexually, with a small mobile sperm and a larger egg. Many can also reproduce asexually.
- Embryonic development: The fertilized egg divides into a hollow ball of cells that rearranges into layers during development.
How is an animal body organized?
Complex animals are organized at several levels. Cells that are similar in structure and function form tissues. Several tissues form an organ, and a group of organs that work together makes an organ system.
The four basic animal tissues are epithelial tissue, which covers surfaces and lines cavities, connective tissue, which supports and joins other tissues, muscle tissue, which produces movement, and nervous tissue, which sends signals. The stomach is a good example of an organ that contains all four. It has an epithelial lining, connective tissue to hold it together, muscle to churn food and nerves to control it.
Simple animals have fewer levels. Sponges have different cell types but no true tissues, and jellyfish have tissues but only simple organs.
What are the types of body symmetry?
Symmetry describes how the parts of an animal’s body are arranged around an axis or a plane.
Asymmetry
Asymmetric animals have no regular shape. Most sponges are asymmetric.
Radial symmetry
A radially symmetric animal can be divided into similar halves along any plane through the central axis, like a pie. Jellyfish, sea anemones and adult sea urchins have radial symmetry (the larva of a sea urchin is bilateral). It suits animals that fix themselves in place or drift, because they meet the environment from all sides.
Bilateral symmetry
A bilaterally symmetric animal has a left and right half that are mirror images. This body plan has a head end, a tail end, a back and a belly, and it is linked to directed movement. Insects, fish, birds and humans are bilaterally symmetric. Concentrating sense organs and the brain at the front, a process called cephalization, is common in this group.
How do tissue layers and body cavities differ?
During early development, animal embryos form layers of cells. Sponges lack true layers. Cnidarians, such as jellyfish, have two layers: an outer ectoderm and an inner endoderm. Most other animals have three: ectoderm, mesoderm and endoderm. The mesoderm gives rise to muscle, bones and many organs.
Animals with three layers are also classified by their body cavity, which is the fluid-filled space between the gut and the body wall.
- Acoelomate: No body cavity, as in flatworms.
- Pseudocoelomate: A cavity only partly lined by mesoderm, as in roundworms.
- Coelomate: A cavity fully lined by mesoderm, called a coelom, as in earthworms, snails, insects and humans.
A coelom allows organs to grow and move independently of the body wall and can act as a fluid skeleton. In arthropods the main body cavity is a modified one called a hemocoel.
Body plans compared
| Animal | Symmetry | Tissue layers | Body cavity | Nervous system |
|---|---|---|---|---|
| Sponge | None or radial | No true layers | None | None |
| Jellyfish | Radial | Two | None | Nerve net |
| Flatworm | Bilateral | Three | Acoelomate | Simple brain and nerve cords |
| Earthworm | Bilateral | Three | Coelomate | Brain and ventral nerve cord |
| Human | Bilateral | Three | Coelomate | Complex brain and spinal cord |
Worked example: classifying an unknown animal
Suppose you find a soft animal on a rock by the sea. It has a ring of tentacles around a central mouth and it sticks to the rock. Use a short series of questions.
- Is it an animal? It is multicellular, has no cell walls and gets food by catching small prey. Yes.
- What is its symmetry? Dividing it along any line through the mouth gives similar halves, so it is radially symmetric.
- How many tissue layers? A radially symmetric animal with a sac-like gut and tentacles is likely to have two layers.
- Which group fits? Radial symmetry, two layers and stinging tentacle cells point to the cnidarians. The animal is probably a sea anemone.
The result is a reasoned conclusion and not a guarantee. Layers cannot be seen from the outside, so a biologist would confirm it from a reference book or from microscope slides. The example shows how few observations are needed to narrow down the group.
How did scientists classify animals?
Aristotle divided animals into those with blood and those without, a rough forerunner of vertebrates and invertebrates. In the eighteenth century Carl Linnaeus created a system of naming and grouping that is the basis of modern classification.
Today classification relies on evidence about evolutionary relationships, including DNA. Some traditional groups have been revised. The older division of animals by body plan still helps beginners, but the evolutionary tree is more detailed.
Why do animal body structures matter?
Body organization affects how animals live and what they need. Understanding it helps in medicine, because many drugs are first tested on animals with similar organ systems. It also guides conservation, since a species with a specialized body plan may be more vulnerable to environmental change.
Engineers use body plans as inspiration for robots. Radial designs inspire soft robots with several arms, and bilateral designs inspire walking and swimming machines.
Which mistakes are common?
Mistake 1: Treating “animal” as the same as “mammal”
Mammals are only one group. Insects, fish, worms and corals are animals too, and most animals have no backbone.
Mistake 2: Assuming all animals can move
Adult sponges and corals are fixed to one place. They are still animals because of their cell structure and their way of feeding.
Mistake 3: Confusing symmetry with shape
A starfish has radial symmetry in its adult form, even though it looks different from a jellyfish. Symmetry describes the arrangement of parts around an axis.
Mistake 4: Mixing up tissues and organs
A tissue is a group of similar cells, while an organ contains several tissues. Muscle is a tissue, and the heart is an organ.