What Is an Animal?

Lesson Plan - Get It!

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Scientists have long studied the differences between living and non-living things, humans and animals, and animals and plants.

Zoologists have developed systems of classification for the various kingdoms of living things. So, if a scientist finds a completely new type of living thing, he or she can quickly decide if it is an animal or not.

According to scientists, animals are multicellular organisms belonging to the kingdom Animalia. They eat organic material, breathe oxygen, are capable of movement, reproduce sexually, and undergo a specific developmental stage, known as the blastula, during embryonic growth.

That's a great summary, but it needs to be broken down to be understood.

Animal Characteristics

Most animals share multicellularity, eukaryotic cell structure, specialized tissues, sexual reproduction, and the ability to move, eat, and possess an advanced nervous system.

Amount of Cells

Animals, plants, and fungi are all multicellular, meaning they have many cells. The number of cells, of course, varies from animal to animal. A horse has 400 trillion cells. A dog has about 50 trillion.

Eukaryotic Cell Structure

The structure of an animal's cells is different from that of non-animals. Animals have a eukaryotic cell structure with a well-defined nucleus and internal organelles.

Eukaryotic cells are also capable of coming together and forming organisms. All animals, plants, fungi, seaweeds, and many unicellular organisms have eukaryotic cells.

This diagram shows the difference between eukaryotic and prokaryotic cells.

Specialized Cells

Animals have highly specialized cells. Ordinary cells called stem cells grow and change into cells designed explicitly for nerves, connective tissues, muscles, and epithelial tissues (which line the organs and blood vessels).

These specialized cells then group together and form various organs.

Sexual Reproduction

Animals need two parents to reproduce. They combine genetics and create offspring with the DNA of both. This reproduction method allows animals to test new gene combinations and develop adaptations to adjust to different environments.

Sexual reproduction is also used by some plants, fungi, and even some bacteria.

Blastula Stage

The explanation of this characteristic can be a bit complicated. Animals go through a stage of development that other life forms do not.

Each animal starts as a zygote or a single cell. The cells divide and form a morula. The next stage is called a blastula. It's a hollow sphere of multiple cells around a cavity of fluid.

During this stage, the cells start differentiating into different tissue types.

Motility

Motility is the ability to move. Other than Venus flytraps, plants (and fungi) can't move.

One of the key characteristics of animals is that they can move around freely, at least at some stage of their development. Some, like sponges and coral, can only move during the early stages, but most animals use movement to find new habitats, follow prey, or escape predators.

Ability to Eat Food

Everything that lives needs carbon to support life. Plants, which get carbon from the environment, are called autotrophs.

Animals ingest other living organisms for carbon; they're called heterotrophs. All fungi and even some plants are heterotrophs also.

Advanced Nervous Systems

Finally, animals are distinguished by their advanced nervous systems, which enable sight, hearing, smell, taste, and touch. Lower animals, such as insects and fish, have a basic nervous system, while the more advanced ones have fully developed brains.

Now that you've learned how to explain an animal, go to the Got It? section to review what you learned.