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  • Hey there, I'm just hanging out with some distant relatives here.

  • 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

  • very skittery.