Vertebrates vs Invertebrates

Hold a freshly caught prawn in your hand and squeeze it gently, it squishes, because there is no hard internal spine, only a shell on the outside. This single question divides every animal on Earth into two groups.

It's easy to think vertebrates are "the main animals" because we see them most, but invertebrates are far more common. There are more species of beetle alone than all the vertebrate species put together.

The split is worth making because of what a skeleton has to do. A skeleton gives muscles something to pull against, and there are only two places to put one: inside the body or outside it. A vertebrate's spine sits inside, built from a chain of small bones called vertebrae, and it is living tissue, so it adds material and gets longer as the animal grows. The invertebrates you meet most often, the arthropods, which is insects, spiders, crabs and their relatives, wear their hard skeleton on the outside instead, and an exoskeleton is not living tissue. Once it has hardened, it cannot get any bigger.

That one difference has a consequence you can watch happen. A crab or a cicada can only grow by climbing out of its old exoskeleton and swelling up before the new one sets, and for those few hours it is soft, slow and cannot hold its own weight. The heavier the animal, the more dangerous that window is, which is why the biggest land animals are all vertebrates.

Vertebrates are split into five groups called classes:

That table is a lot to hold in your head at once, and the useful news is that you almost never need all of it. Each class has at least one feature that no other class has, and finding that feature settles the class on its own. Feathers are the clearest case: birds are the only animals on Earth that grow them, so spotting a feather is a complete answer and you can stop looking. Fur together with milk does the same job for mammals.

The other three classes are separated by their skin, and it is worth seeing why skin carries so much weight. Skin is where an animal loses water. An amphibian's skin is thin and moist because the animal partly breathes through it, and thin moist skin dries out fast, so a green tree frog has to stay near a creek, a dam or a damp shed. A reptile's dry scales are waterproof, and that single change is what let reptiles move into country a frog cannot survive in. It is not a coincidence that the bearded dragon lives in the desert and the frog lives by water: the skin decides where the animal can go.

Now the trap this shortcut walks into. Where an animal lives is not its class. An olive sea snake spends its whole life on the Great Barrier Reef and never comes ashore, and it is a reptile, because it has dry scales and has to surface to fill its lungs with air. Habitat tells you what an animal has had to cope with. Only its body tells you what class it is in.

Invertebrates are split into many groups. Here are the five biggest you should know:

A handy spider/insect rule: count the legs. 6 legs = insect; 8 legs = arachnid. So a Sydney funnel-web is an arachnid, not an insect.

Where counting legs stops working. Turn over a damp log in the garden and you will find a slater, grey, armoured and about a centimetre long. It has a hard exoskeleton like an insect and it is not one, and it is not an arachnid either: it has more than eight legs, and it is a crustacean, in the same group as a prawn and a yabby. It even breathes with gills, which is the reason it dies within hours in the open sun and why you only ever find it somewhere damp. So count the legs second, not first. Ask how many legs there are at all before you assume the animal is one of the two groups the leg rule is about.

A dichotomous key ("di" = two, "chotomy" = cut) is a series of yes/no questions that ends with the species name. At each step you have two choices. Here's a simple example for a few Aussie animals:

To use the key, start at step 1 and follow the yes/no path. You always end at one species. Scientists use much longer keys (hundreds of steps) to identify things like leaves, fish or insects.

Every question in a key has to pass two tests, and both come from the same requirement: the key has to work for somebody who has never seen this organism before. The first test is that the question must have exactly two answers. "Does it have feathers?" splits everything alive into two piles. "What colour is it?" does not split anything, because there are hundreds of answers and no branch to follow. The second test is that you must be able to answer it by looking at the specimen in front of you. "Is it rare?" is a perfectly good scientific question and a useless key question, because the answer is not written anywhere on the animal.

Watch what the key above actually does with a green tree frog. Step 1 asks about a backbone: yes, so go to step 2. Step 2 asks about feathers: no, go to step 3. Step 3 asks about fur and milk: no, go to step 4. Step 4 asks about dry scales: no, and the "no" branch is the frog. Notice what did not happen. The frog was never picked out by something it has. It was picked out by being the last one standing after four other things were ruled out. That is the whole idea of a key: it does not describe an organism, it eliminates every other one.