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In this Nature League Lesson Plan, Brit introduces the invertebrates of Earth, including their names, relationships, forms, and awesomeness.

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Welcome back to Nature League! We've recently explored biodiversity, and discussed the various ways in which we value and define it.

But here's the thing- if we really want to dig into biodiversity, we should probably continue by talking about invertebrates; after all, they make up close to 95% of all animal species on Earth. Who run the world? Bugs. [CHEERY INTRO MUSIC].

When we think about life on Earth, it's easy to just think about vertebrates, or animals with vertebral columns. I mean, those are the animals we see most easily and most often, right? And they resemble us more than the creepy crawlies.

But the thing is, invertebrates make up the majority of species in the animal kingdom. Let's check out this tree that shows the different groups of animals. See the chordate group?

That's where vertebrates are. And that means everything else you see in the animal kingdom are invertebrates. So what is an invertebrate technically?

Spoiler alert, it's not just insects! Invertebrates are technically defined as organisms without backbones, but what they are in practice is widely varied. Invertebrates live in almost every environment on our planet, and can be many different sizes and shapes: everything from microscopic and spiny to massive and gelatinous.

In fact, there are so many types of invertebrates that I think we should take a quick tour of their phylogenetic tree. A phylogenetic tree shows the evolutionary relationships between species and their common ancestors. So for animals, there was a common ancestor that gave way to the evolution of all other animals on Earth, and a phylogenetic tree can let us visualize the different lineages of species.

Our first stop on the invertebrate tour? Let's check out sponges! Sponges are in the phylum Porifera, and they are considered basal animals.

This means their lineage is the closest to the original common ancestor. Because of this, sponges aren't too fancy. They lack true tissues, and are sedentary, meaning they don't really move on their own.

But don't count sponges out! They're super cool invertebrates in their own right, and really useful in terms of human health. Sponges provide many compounds for human antibiotics, especially for penicillin-resistant strains of certain bacterial infections, and researchers are currently working on using sponge-derived compounds to fight cancer.

Moving right along the tree we find our next big split. This branch has to do with body symmetry, and the two groups are animals with radial symmetry and animals with bilateral symmetry. Radial symmetry is when you cut the body in several slices, like a pizza, and get something similar (but don't try this at home).

Think jellyfish, for example...there are no clear left and right sides, just a top and bottom. Bilateral symmetry is when the body is symmetric on one plane. Basically, the body has a clear left and right side.

Humans are a great, familiar, example of this. Let's check out some invertebrates with radial symmetry! These include the cnidarians and ctenophores.

The silent “c” stands for “cool”, because who doesn't love jellyfish. The cnidarians include corals, jellies, and hydras, and ctenophores include comb jellies. Each of these groups have true tissues but radial symmetry.

So that leaves us with the rest of the invertebrates: they all have true tissues and bilateral symmetry, with a couple of exceptions. The next split in the tree is between protostomes and deuterostomes. But what they really mean is what happens in development.

Protostomes have something called a blastopore turn into the mouth, and deuterostomes have that blastopore turn into the anus. You can always remember it because deuterostomes remind me of “doody” which reminds me of “butt.” [BRIT LAUGHS TO HERSELF]. Now besides having nice mnemonics for not forgetting those differences, what's interesting is that this split moves chordates, like us, and echinoderms, like starfish, away from the rest of the invertebrates.

Kind of neat that some of our closest animal relatives look like this. So now that things with chordates and sea urchins have been sorted out, and we've left behind our jellies and sponges, we're still left with a crazy amount of invertebrates. There's a massive group called Lophotrochozoa which includes a ton of smaller groups like flatworms, rotifers, bryozoans, brachiopods, segmented worms, and mollusks like clams, snails, squids and octopuses.

All of this, and we haven't even touched on insects yet. And don't you worry...we're totally going there next. Ecdysozoa are the last group we need to discuss, and they happen to be the most species diverse group of animals on Earth.

This group includes nematodes, or roundworms, and arthropods, which include insects, crustaceans, and arachnids. More than 1,000,000 species of arthropods have been described by scientists so far, and these are just the ones we know about! The majority of arthropods are insects- in fact, insects are more species-rich than all other forms of life combined.

The majority of insects are winged insects, which include beetles, flies, mosquitoes, ants, bees, wasps, butterflies, moths, true bugs, grasshoppers, crickets, and others. If you're overwhelmed by all of the groups of species we've covered in the last few minutes, it's kiiiiiinda the point. Invertebrates make up the overwhelming majority of animal species on Earth, and we may as well give them some respect.

Speaking of respect, invertebrates make up a gigantic amount of biomass, and that means they significantly affect the landscape, water, atmosphere, and other lifeforms here on Earth. They consume significant amounts of plant matter, other animals, and cycle nutrients by decomposing materials. They serve as food for larger organisms and even make up the majority of protein in some human diets.

And just think!! Invertebrates pollinate crops, which is fundamental to our well-being! Now, in this Lesson Plan, we've only scratched the surface of the complex relationships and diversity of invertebrate species.

Phylogenetic trees are constantly changing based on new information, so some of the branches on the tree we've discussed here very well may change. One thing is clear though- when it comes to the animal kingdom, invertebrates are where it's at. Thank you for watching this episode of Nature League.

Make sure to subscribe at, and we'll see you next week for an invertebrate Field Trip. Nature League is a Complexly production. Want more invertebrates?

Click the suggested video to watch a video by our sister channel SciShow about some amazing structures built by bugs.