What is zoology?
Zoology is the science that studies animals: how they look, how they live, what they eat and how they are connected to each other. A person who studies animals is a zoologist.
There are more than a million kinds of animals known. Zoologists sort them into groups so that they are easier to learn.
How animals differ from plants
- Food: Plants make food from sunlight, water and air (photosynthesis). Animals cannot. They eat plants or other animals. We say animals are heterotrophs (hetero = other, troph = feeding).
- Movement: Most animals can move their whole body from place to place. Plants only bend or grow.
- Cell wall: Plant cells have a stiff wall. Animal cells have only a thin soft covering, so animals can be flexible.
- Growth: A plant keeps growing new parts all its life. Most animals grow to a size and then stop.
- Senses: Most animals have nerves and sense organs to react fast. Plants react slowly.
Some living things are in-between. A sea sponge looks still, but it is an animal because it cannot make its own food.
One cell or many cells
A unicellular animal has just one cell. This one cell does everything: eating, moving, getting rid of waste. Amoeba and Paramecium are examples. You need a microscope to see them.
A multicellular animal has many cells. Different cells do different jobs. Muscle cells move the body, nerve cells carry messages, gut cells digest food. Your own body has trillions of cells. Cells of one kind form a tissue, tissues form an organ, and organs working together form an organ system.
Symmetry: the shape of the body
Imagine a mirror line cutting an animal into two halves. If the two halves look the same, the animal has symmetry.
- Asymmetry (no symmetry): No line makes two matching halves. A sponge is like this.
- Radial symmetry: Many lines through the centre make matching halves, like the slices of a pizza. A starfish, a jellyfish and a sea anemone are like this. They can meet food from any side.
- Bilateral symmetry: Only one line (from head to tail) makes matching left and right halves. A fish, a dog, a butterfly and a human are like this. They have a front end (head) and a back end, and they usually move forward.
Try it
In the 3D: go to the last step, press Sponge, Starfish and Fish one by one and watch the red mirror line. Where does it fit? Where does it not?
At home: fold a paper in half and draw half a butterfly along the fold. Cut it out and open it. You have made a bilateral shape. Now look at a flower or a sliced lemon from the top. Is it radial? Guess first, then check.
Key formulas and definitions
- Zoology = the study of animals
- Animal = eats ready-made food (heterotroph) + usually moves + most have many cells
- Unicellular = 1 cell (Amoeba). Multicellular = many cells (dog, fish)
- Radial symmetry = many mirror lines (starfish). Bilateral symmetry = one mirror line (fish, human)
- Cells โ tissue โ organ โ organ system โ organism
Worked examples
1. A sea sponge sits on a rock all its life and does not move. Is it a plant or an animal?
It is an animal. It cannot make its own food. It filters tiny food bits out of the water. Not moving does not make something a plant.
2. Classify these by symmetry: jellyfish, butterfly, sea anemone, human.
Jellyfish and sea anemone are radial. Butterfly and human are bilateral.
3. Why can a unicellular animal live without organs?
One cell does every job itself. It takes in food, gets energy and removes waste. A big body with many cells needs special organs to share the work.
Common mistakes
- Thinking that anything that does not move is a plant. A sponge or coral is an animal.
- Mixing up radial and bilateral. Radial means many matching lines. Bilateral means only one.
- Saying all animals have many cells. Amoeba has only one.
- Saying animals make their own food. Animals eat ready-made food.