Invertebrates
Every animal you can think of probably has a backbone: fish, frogs, birds, even you. That group is actually the minority. This lesson is about the animals that make up almost all of the rest.
What Do 9 Out of 10 Animals On Earth Have In Common?
None of them have a backbone. Invertebrate is the umbrella term for any animal with no vertebrae running down its body, and jellyfish, octopus, worms, insects, and crabs all qualify, even though they look nothing alike.
There's no automatic advantage to having one. A backbone helps some animals grow large on land and move in specific ways, but invertebrates solved the same problems (support, movement, defence) using exoskeletons, water buoyancy, and body plans that never needed a spine at all. The rest of this lesson breaks down exactly how.
Put Your Instincts to the Test
Think about what you already know about animals without backbones. Pick an answer for each question, then see if your instincts were right.
The claw grows back gradually across several molts. The episode tracks one crab through three separate shell sheddings: first the claw is missing entirely, then small and new, then nearly full size. Regeneration happens in stages tied to the molting cycle, not all at once.
No, it collapses under its own weight. A water balloon demonstrates this well: held in the air it folds in the middle, but lowered into water it straightens out completely because the water supports it. It's exactly why the largest invertebrates on Earth, like giant squid and octopus, live in the ocean rather than on land.
No, the signal can travel directly to a nearby body part through a nerve net. Sea jellies have no backbone and no central nervous system, so their nerves are spread evenly through the body instead of routed through one control centre. A stimulated part of the net can pass the signal straight to a neighbouring part without ever reaching a brain, because there isn't one.
Understanding the Science
Tap each card to reveal the explanation, then try the two labs below.
Key Concepts
What Makes an Invertebrate
Tap to learn moreAn invertebrate is any animal without a backbone, or vertebrae. It's a strange dividing line: animals like you, fish, and frogs all sit on one small branch of the tree of life, while roughly 9 out of 10 animal species sit on the other. Everything in this lesson comes down to how that second, much bigger group manages to move, grow, sense, and defend itself without a spine.
A World of Invertebrate Groups
Tap to learn moreInvertebrates aren't one body plan, they're dozens. Arthropods (crabs, insects, spiders) wear a hard shell, mollusks (squid, octopus, snails) are mostly soft, cnidarians (jellies) are little more than a nerve net and a stomach, and worms and sea stars each solve the no-backbone problem their own way. That variety is exactly why invertebrates dominate every environment on Earth.
Exoskeletons: Armor From the Outside
Tap to learn moreWithout an internal skeleton, many invertebrates grow a hard exoskeleton on the outside instead, giving muscles something rigid to pull against. The catch is that it can't grow along with the animal inside it, and it doesn't scale up to any size either. That trade-off is exactly why exoskeleton animals have to molt.
Molting: Growing By Shedding Your Skeleton
Tap to learn moreTo grow bigger, an exoskeleton animal has to shed its old shell entirely and grow a new, larger one underneath, a process called molting. For a short window afterward, the animal is soft and exposed before the new shell hardens. The episode follows one crab through several molts as it slowly regrows a lost claw.
Regeneration: Regrowing What's Lost
Tap to learn moreMany invertebrates can regrow a lost limb, something no vertebrate can do. A crab that loses a claw in a fight can regrow it fully, but only in stages, a little more with each molt. Sea stars can do the same trick with an entire arm.
Nerve Nets: Thinking Without a Brain
Tap to learn moreInstead of a brain and spinal cord, many invertebrates run on a nerve net: nerves spread evenly through the body with no central control point. A touch on one side can trigger a reaction nearby without the signal having to travel all the way to a brain first. Try the race in the lab below to see the difference for yourself.
Body Support Without a Backbone
Tap to learn moreA backbone doesn't just protect your spinal cord, it holds your body up against gravity. Soft bodied invertebrates don't have that, so on land they collapse under their own weight past a certain size, but in water, buoyancy does the job instead. That's the real reason giant squid and octopus live in the ocean and not on land.
Invertebrates as Ecosystem Engineers
Tap to learn moreA huge share of invertebrates spend their whole lives eating dead material: fallen leaves, dead wood, dead animals. The episode's forest pitfall trap turns up dozens of these decomposers doing exactly that job. Without that constant cleanup, dead matter would simply pile up.
Reaction Race: Nerve Net vs Brain and Spinal Cord
Click anywhere on either creature to fire a signal. Most of the time, brain and spinal cord wins, it's the faster system overall. But watch for the glowing ring around the target: click inside it and something different happens.
How Fast, Really?
A jellyfish's nerve net sends a signal at about 0.2 metres per second, roughly the speed of a slow hand wave. A brain and spinal cord pathway conducts faster once it gets moving, but it always pays a small, fixed processing cost first, a few thousandths of a second, before it can react at all. Right next to the target, that fixed cost is all that matters, which is why the nerve net can win inside the glowing ring. Anywhere outside it, the brain and spinal cord's raw speed takes over and wins easily.
A plain nerve net, like the one in this race, is slow over any real distance. But some invertebrates build a completely separate shortcut circuit for when they truly need speed. A crayfish flicking its tail away from a hungry predator reacts in less time than it takes you to blink, one of the fastest reflexes of any animal on Earth.
Apply Your Knowledge
Invertebrates solve the same problems vertebrates do (support, movement, growth, and defence) but every group does it differently. Match each strategy to what it actually solves.
Match the Concepts
Click an object to select it, then click the matching description to place it.
Real-World Challenge
A marine biology team just discovered a new species of soft bodied invertebrate living exclusively in shallow tide pools that are exposed to air twice a day at low tide. Based on what you have learned about exoskeletons and body support without a backbone, what physical challenge would this animal face at low tide that a similar species living permanently underwater would not, and what adaptation might help it survive being exposed to air?
What Has Changed Since This Episode Aired
This episode first aired in 1996. The core science about invertebrates still holds up well, but research since then has sharpened a few of the specific numbers and ideas it used.
Updated: Refined surveys of animal diversity now put the figure higher, around 95 to 97% of all animal species are invertebrates, according to the IUCN and the American Museum of Natural History. Arthropods alone (insects, crustaceans, spiders, and their relatives) make up the overwhelming majority of that number. The core point still holds, animals with backbones are a small minority, and it has only gotten stronger as scientists have described more species.
Updated: Not anymore. In 2023, researchers at Kiel University and the University of Copenhagen, publishing in the journal Current Biology, trained Caribbean box jellyfish, which have no central brain and only about 1,000 neurons total, to associate a visual pattern with an obstacle and change their swimming behaviour to avoid it. That is a real form of learning called associative learning, something scientists had assumed required a centralised brain. The nerve net's wiring is still described accurately here, but scientists now know a decentralised nervous system can do more than simple reflexes.
Updated: Not exactly. Genetic research over the last few years has shown that regrowing a limb is a tightly controlled biological program, not a simple default. When a crab or crayfish loses a leg, specialised cells gather at the wound site to form a structure called a blastema, and a specific set of signalling pathways, including ones named Wnt and TGF-beta, directs that blastema to rebuild the limb in the correct shape, timed to the animal's molting cycle. The reason invertebrates regenerate more easily than we do isn't that their bodies are simpler, it's that they run a specialised repair program most vertebrates lost over the course of evolution.
Test Your Understanding
Answer these questions and get instant feedback. How many can you get right?
Results
Your score:
Reflection
This lesson keeps circling back to one idea: invertebrates solve every problem (support, growth, defence, sensing) differently than we do. Which strategy surprised you the most, and can you think of a human made technology that solves a similar problem in a similar way, for example armor, hydraulics, or a distributed network?
Episode Discussion
Share your thoughts on this Bill Nye episode