I've got to say I've got a lot of respect for these guys,

because they are some of the most successful organisms on the earth.

We think we run things on this planet, but we don't.

They do.

On one acre of cultivated land, annelids like this earthworm

process about 16,000 pounds of soil a year, which makes plant life

and our lives

therefore, possible.

And not only are there about a billion billion arthropods

like this in the world, that's 10 to the 18th power by the way,

but also 80% of known animal species are arthropods.

Now, don't get me wrong, even though bugs and spiders

and worms and shrimp totally outnumber us humans,

we are far more complex than them.

When it comes to arm wrestling and guitar playing

and long division, we'd totally school them.

But complexity like ours had to start somewhere.

And it started with a very special trait

that we share with these animals.

Can you see the resemblance?

Last time we talked about how, in the very simplest animals,

there are two traits that indicate an animal's relative complexity:

there's how many germ layers they develop when they're embryos

and whether or not they have a coelom, or body cavity

that holds their organs.

So it's in the next two phyla, Annelida and Arthropoda, where we

find the new big thing in animal complexity: segmentation.

Segmentation is the repetition of anatomically identical units

that can be added to and modified to serve different purposes

as animals evolve.

And evolution is the way to win it, folks.

In fact, the three biggest and most diverse groups of animals

in the world are the ones that display segmentation:

Annelida, Arthropoda and Chordata, which includes the vertebrates.

All segmented animals have a common ancestor

that probably lived about 600 million years ago.

That's how long ago it was when one of your grandparents

and one of the earthworm's grandparents

and one of the beetle's grandparents

all played on the same softball team.

Pretty crazy.

Segmentation has proven to be unbelievably useful

from an evolutionary perspective.

In humans, you see anatomically identical pieces repeated

along an axis from our butts to our heads.

They can be a little hard to see because they're so highly modified,

but think about our vertebrae: They're segments!

Our ribs are segments!

The cartilage around our trachea? Those are segments!

Even the folds in our brains are segments.

They're crazy-evolved segments, but segments just the same.

Among today's animal phyla, the earliest to display segmentation

is Annelida, which includes leeches, earthworms and lugworms.

See how their whole bodies look like rings fused together?

Segments!

In fact, the word Annelida comes from the Greek for "little rings,"

and when you look at any annelid, you see that they're all

really obviously segmented.

Now, this segmentation is a great example

of synapomorphy in annelids.

Synapomorphies are traits that set one group of animals apart from its

ancestors and from other groups that came from the same ancestors.

So unlike their flatworm and nematode cousins,

Annelids are segmented and they've also got little bristles

on their bodies called chaetae that provide traction

and help them move through the dirt.

These are both little extra somethings that annelids' have,

that less complex relatives don't have and that their common

ancestors didn't have.

Synapomorphies, literally "shared derived traits,"

are usually the defining traits of a phylum.

But you can also learn something about an animal's lineage

by comparing plesiomorphies, very basic traits

that are shared by animals with a common ancestry.

So, between the Platyhelminthes, the Nematodes and the Annelids,

one plesiomorphy is that they're all worm-shaped,

which tells you they have a common, distant, ancestor

that was wormy-lookin'.

So as we talk about these phyla and the classes within them,

notice how they're similar and different from each other.

For instance, within Annelida, there are three different classes.

Everybody's favorite, of course, is the oligochaetes, the earthworms

Their name refers to the synapomorphy I just mentioned:

they have "chaetae", or bristles,

but only an "oligo", few of them.

And they're everyone's favorite because they eat soil and crap

it out the other end, allowing air and water

to circulate in soil.

Plus their poo is rich in things that plants need to grow,

like nitrogen, calcium, magnesium, and phosphorus.

And now i'm going to go wash my hands.

Now on to the class Hirudinea, the leeches,

a lot of which are parasitic and, you know, eat

blood and other bodily fluids.

A synapomorphy of leeches are their powerful suckers

they've got them on both ends of their body,

the posterior one being used to anchor itself while the

anterior one that surrounds its mouth attaches to its host.

All leech species are carnivorous and they are hermaphrodites

like earthworms.

Now, the Polychaetes are bristly worms, hence a synapomorphy

of this class is their "poly" (many) "chaete" (bristles).

Almost all of these are marine species,

and they're really diverse, but the ones you've probably seen

evidence of are the lugworms, the ones that dig holes

at the beach and leave piles of castings on the sand.

Okay, I know you've had enough of worms.

Now we've got Arthropoda to talk about, and that's not

very easy to do because there are A LOT of them.

Like I said, they totally outnumber us.

Just to put things into perspective, there are more

insects in a square mile of rural land than there are

human beings on the earth.

One reason scientists think arthropods do so well has to

do with their—

you guessed it!

-segmented bodies.

Fossils of early arthropods show that there used to be

very little variation between segments, but as they evolved,

segments fused and became specialized for different

functions, which led to crazy amounts of diversity.

So much diversity that Arthropoda includes stuff like scorpions,

butterflies and lobsters.

Which...I know.

Seems like maybe a bit of a stretch.

But here are the synapomorphies that make them all arthropods:

1. They have segmented bodies that, in most cases, are broken up into

three segments: head, thorax and abdomen.

And check this out: Here, segmentation in arthropods

is a good example of a plesiomorphy.

It's a basic, ancient trait that they share

with annelids and us chordates, for that matter,

dating back to that softball game our forebears played

some 600 million years ago.

2. All arthropods have an exoskeleton: a hard outer shell

made out of chitin, which is a really tough carbohydrate that's

chemically similar to the cellulose that you find in plants,

and in order to grow bigger they have to shed it.

And be glad that you don't have to do that.

Because it looks like kind of a traumatic experience for them.

3. Finally, they've all got paired and jointed appendages,

which is actually where their name comes from:

arthropod means "jointed feet."

But it's not just their legs that are jointed. Some of them have

claws and jointed antennae, and they all have these

external mouthparts that are also jointed.

So that's what all arthropods have in common, but they are

grouped into 4 subphyla, based on how they differ

from each other.

First, and perhaps most terrifying, you have your Cheliceriformes,

which includes spiders and scorpions, but also

horseshoe crabs which are kind of nice,

and ticks, which aren't

and mites, which don't bother you at all,

probably...

they might.

Cheliceriformes comes from the Greek for "arm-lips," which--

Whatever, Greeks...

refer to their long fang-like pincers.

Unlike a lot of arthropods, they have simple eyes with

just one lens, not compound eyes like flies, and they lack antennae.

Most Cheliceriformes are landlubbers, but the fossil record

tells us that a lot of them were marine back in the olden days.

Sea spiders and horseshoe crabs are the only ones left now.

The largest class of Cheliceriformes are

the arachnids, the group that includes spiders, scorpions,

ticks and mites.

They have what's called a cephalothorax, which is a

head segment and a thorax segment fused together,

with eight legs and an abdomen behind.

Most arachnids are carnivorous or parasitic and they're