Some living things swim across whole oceans. Others spend their entire life glued to one rock. Both of them move — just in very different ways. This page sorts out the two words the examiner wants you to use, motile and sessile, and clears up the trap that sits between them.
📘 What you need to know
Movement is one of the things every living organism does. Nothing alive is completely still inside.
Motile organisms can move their whole body from one place to another.
Sessile organisms stay fixed in one spot, but they can still move parts of themselves.
Motile organisms do both kinds of movement — they travel around, and their insides move too.
Sessile does not mean dead, frozen or lazy. It only means the whole body does not change position.
You must be able to name one motile species and one sessile species in the exam.
What counts as movement?
Biologists use “movement” in a wide sense. It covers a cheetah sprinting, but it also covers a sunflower head turning, cytoplasm circling inside a single cell, and food being squeezed along your gut while you sit still reading this.
So when a question asks about movement, do not jump straight to legs and muscles. Ask yourself first: is the whole organism changing position, or is only a part of it moving? That one question splits every organism into the two groups below.
Motile organisms
A motile organism can move itself from place to place. It picks up its whole body and relocates — by swimming, walking, flying, crawling or being whipped along by a flagellum.
Nearly all animals are motile. Think of a shark, a locust, a snail, or you.
Plenty of bacteria are motile too, especially the ones that hunt other microbes. They use a flagellum like a tiny propeller.
Motile organisms move for a reason: food, safety, a mate, or better conditions.
Here is the bit students forget. A motile organism also shows the other kind of movement. Your heart squeezes, your gut pushes food along, your pupils change size. Being motile adds an ability — it does not remove one.
Sessile organisms
A sessile organism is anchored. It cannot pick itself up and relocate. But it is far from motionless: parts of it move all the time, usually in response to something in the environment.
A plant stem bends towards the light. The plant has not gone anywhere, but it has moved.
Cytoplasm streams around inside a single cell, carrying material with it.
A sea anemone waves its tentacles and pulls food into its mouth.
Sessile organisms you can name: sponges, corals and sea anemones, most fungi, and all plants.
The fish changes address. The plant changes shape. Only one of them is motile.
If you can point to a spot where the organism is stuck down — roots, a holdfast, a base cemented to rock — you are looking at a sessile organism.
The overlap that catches people out
Draw it as a nesting, not a split. Every organism can move parts of itself. Only some can also move the whole body. So motile organisms have both abilities; sessile organisms have one.
The rule in one line
motile = moves parts and moves the whole body • sessile = moves parts only
Peristalsis is the easiest example to remember, because it happens in you. Circular muscle behind a lump of food squeezes shut and pushes it along the gut. You are motile, but that particular movement is the sessile kind — a part moving inside a body that is going nowhere.
Your gut does this right now while the rest of you sits still. It is the same class of movement a sessile organism relies on.
Side by side
Feature
Motile organism
Sessile organism
Whole body moves place to place
Yes
No
Parts of the body can move
Yes
Yes
Named examples
Jellyfish, beetles, humans, predatory bacteria
Sponges, corals, sea anemones, fungi, all plants
A movement it shows
Swimming off to find food
A stem bending towards the light
Learn one of each and stick with it. A jellyfish for motile and a coral for sessile will answer almost any version of this question. Do not invent a new example under exam pressure.
Worked examples
WORKED EXAMPLE
Name one motile species and one sessile species, and state the difference between them. [3]
Give a named example for each
Motile: the moon jellyfish. Sessile: the sponge.
Now state the difference in the right wordsThe jellyfish can move its whole body from one place to another; the sponge stays fixed to one spot.Add the detail that earns the third markThe sponge can still move parts of its body in response to stimuli“a plant” or “an animal” is not a species — name something specific.
WORKED EXAMPLE
A sea anemone is attached to a rock. Its tentacles wave and drag food into its mouth. Is it motile or sessile? Explain. [2]
Ask the one question that matters
Does the whole body change position? No — it stays attached to the rock.
So the waving tentacles do not make it motileMoving parts of the body is something sessile organisms do too.Sessile — it is anchored, and only parts of it movethis is the classic trap: movement of a part is not locomotion.
💡 Exam tip
Have one named motile species and one named sessile species ready before you walk in. That is exactly what the syllabus asks for.
Use the words motile and sessile in your answer. “It moves” and “it does not move” will not score.
If a question describes an organism doing something, check whether the whole body shifts. That is the deciding test.
For sessile organisms, always add “but it can still move parts of its body in response to stimuli”. It is often a separate mark.
Link movement to a stimulus where you can — light, touch, chemicals. Examiners like cause and effect.
⚠ Common mix-up
Thinking sessile means motionless. A sessile organism can be busy. It just cannot relocate.
Thinking motile organisms only do whole-body movement. They do both kinds. Peristalsis is the proof.
Using “sessile” for anything that is slow. A snail is slow and fully motile.
Confusing sessile with “sedentary”. Sessile is fixed in place; sedentary just means it does not move much.
Naming a group instead of a species. “Fungi” is a kingdom, not an example.
Forgetting bacteria. Many are motile, and they are a neat example if you want to stand out.
Up next: How Muscles Contract — we zoom right in on the machinery that lets a motile animal pull itself around.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
