Animals 101

GeoZoo’s Introduction discusses life. Biologists organize living things into kingdoms. GeoZoo focuses primarily on the animal kingdom, Animalia. But what are animals?

Most people can easily distinguish vertebrates (mammals, birds, reptiles, amphibians and fish) and just about any invertebrate that walks, crawls or swims from plants and other living things. However, things get a little more confusing when we consider primitive animals that are sessile (that is, anchored to the ground - usually the sea floor), really tiny animals (especially species that live as parasites inside our bodies) and animals that just don’t look like animals (including many sessile and really tiny species).

What separates animals from plants and bacteria?

Consider this definition from Wikipedia’s article, Animals:

Let’s see if we can make that a little easier to understand...

1) Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa.

The word muticellular simply means animals are made of many cells, as opposed to unicellular organisms. Eukaryotes are organisms (mostly plants and animals) whose cells have nuclei (singular = nucleus). A bacteria cell doesn’t have a nucleus; thus, bacteria and archaea are classified as prokaryotes. (Note: Most prokaryotes are unicellular, but some are multicellular.)

Does that still sound a little confusing? OK, let’s make it simpler still: If it’s a living thing, it isn’t a plant, and it’s bigger than a bacterium, it’s probably an animal - or a fungus, but fungi resemble plants more than animals.

It’s easy to remember that animals belong to the kingdom Animalia. We’ll learn more about animal classification in the article Animal Classsification.

2) Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. (This is probably supposed to read early in life.)

Trees can continue growing bigger and bigger, with more branches appearing. Thus, their appearance can change over time.

But animals don’t continue growing legs, wings, fins or tails. All living flying animals have one or two pairs of wings - no more. No vertebrates have more than four legs.

However, some animals change form early in life. For example, frogs lay eggs which hatch into tadpoles - tiny creatures with tails but no legs. As they mature, legs appear, and the tails disappear. Butterfly larvae go through an even more complex process as they transform from caterpillars into beautiful butterflies.

This process is called metamorphosis. (The verb form is metamorphose.) Mammals, birds and reptiles don’t exhibit metamorphosis. Many amphibians, some fish species and many invertebrates do.

> Learn more about animal body plans in Morphology (Anatomy and Appearance) and more about metamorphosis in Reproduction.

3) Most animals are motile, meaning they can move spontaneously and independently.

There are two kinds of voluntary animal movement. The first involves moving a part of one’s body - opening one’s eyes, flicking your tail, etc.

The second involves moving one’s entire body from one place to another, a process called locomotion - at least in bigger animals. In vertebrates, locomotion is usually accomplished by walking (or running), hopping, crawling, climbing, swimming or flying.

However, not all animals are capable of locomotion; for example, corals and sponges are sessile; in other words, they are rooted to one spot, similar to plants.

> Learn more about Animal Locomotion.

4) All animals are also heterotrophs, meaning they must ingest other organisms or their products for sustenance.

In comparison, most plants are autotrophs, capable of obtaining food and water directly from air and soil. However, there are a few plants that eat animals, including the Venus flytrap and pitcher plants.

> Learn more about Animal Diets.

Simple Definitions

The following definitions are simple and not precise. However, they may help you understand what animals are.

1. Animals are living things that obtain oxygen from air or water and derive nutrition from other organisms - living, dead or decaying. Most are capable of locomotion.

2. Just about any living thing that exhibits locomotion and can be seen with the naked eye is an animal (though a few animals aren’t capable of locomotion, and many invertebrates cannot be seen without a microscope).

3. Animals = mammals, birds, reptiles, amphibians, fish, insects, crustaceans and just about anything that can be described as a shellfish, worm or marine invertebrate.

And here’s a tip regarding sessile benthic organisms - creatures that live on the bottom of the sea (benthic) and aren’t capable of independent locomotion (sessile). Crinoids, commonly known as sea lilies, are a classic example. Are they plants or animals?

In fact, few plants are adapted to saltwater. Moreover, a lack of sunlight makes it almost impossible for them to grow in deeper water. Thus, marine plants are generally limited to species growing near the shore or floating at the surface.

So most of the bizarre creatures explorers see in the deep sea are animals.

Vocabulary

In the What is Life? we learned that the Greek word bio means life. It is the root of words like biology (the study of life), biotic (living) and abiotic (abiotic).

Now let’s meet an even more familiar root: zoo. You can probably guess what it means - animals. The following words - all nouns - should be pretty easy to remember.

What is an animal again?

The word animal can be a little confusing. Some people don’t regard themselves as animals. Others apply the word animals to mammals.

The words creature and beast are roughly synonymous with animal, though they’re seldom applied to humans. There are various definitions, but beast is often applied to animals (primarily mammals) that are considered fierce or big and powerful. For example, elephants and oxen can be used as beasts of burden, and the lion is commonly referred to as king of the beasts.

People who are big and strong or who are passionate, wild or violent are often described as beasts. (He was a beast on the dance floor.) But creature is often used to describe people who are weak, helpless or in a bad situation. (The poor creature had no money for food.)

The word fauna is more precise; it means animals. A book titled Great Plains Flora and Fauna would discuss plants (flora) and animals (fauna) that inhabit the Great plains.

Series: Animals 101 (12 articles)