Hey there, just hanging out with some of my distant relatives here and 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, like, 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? 

(Intro)

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 a 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
(1:25) 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 and 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 an annelid, you see that they're all really obviously segmented. 

Synapomorphies & Plesiomorphies
(2:44) 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 that 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. 

Annelida
(3:38) 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 and calcium and 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.

Arthropoda
(4:52) Okay, I know you've had enough of worms. And now we've got Arthropoda to talk about, and that's 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 actually 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.

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.

Chelceriformes
(6:41) 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 and ticks and mites and 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. Just sayin'. 

Myriapoda
(7:40) Next, Myriapoda, or "many feet," includes what you would expect: the millipedes and centipedes, these are where the arthropods were like "let's see how far we can take this segmentation thing, shall we?" All Myriapods are terrestrial and have antennae and sort of scary jaw-like mandibles. Millipedes are vegetarians, and they may have been some of the very first animals to live on land, where there were mosses and primitive vascular plants for them to munch. Also, although millipedes have a crap-ton of legs, they don't have as many as a thousand, as their name implies. They've actually got anywhere between 94 and 394 legs, depending on the species. Centipedes, whose name is a little more apt as they generally have between 20 and about 350 legs, are carnivores and have poisoned claws to paralyze their prey. So, if you're looking to cuddle with a Myriapod, I'm going to advise you to go with a millipede. 

Hexapoda
(8:29) But please, save some love for Hexapoda, meaning "six feet," most of which are insects. The synapomorphies they share are three-part bodies, consisting of a head, a thorax, and an abdomen, three pairs of jointed legs that come off the thorax, compound eyes, and two antennae. Now, think of any random way you could put these characteristics together, and you'll probably come up with something that exists. Because, you guys, there are so many damn insects out there you have no idea. There are more species of insects than there are all other animal species COMBINED. Which is why I'm taking this opportunity to do a BIOLO-GRAPHY, The Insects Edition!

Bio-lography: Insect Edition
(9:10) Very little is understood about the evolution of insects, but scientists think that they probably split off from their crustacean cousins about 410 million years ago. And for tens of millions of years, insects and some little skittery invertebrates were about the only land dwelling animals. About 320 million years ago, thanks to the high oxygen levels of the Carboniferous period, some insects grew to be terrifyingly big, like the Meganeura, which looked like a dragonfly with a two-foot wingspan. But since an insects' size is restrained by its respiratory systems, as oxygen levels went down, these massive insects couldn't circulate enough oxygen to keep their gigantic bodies going, and they died off. The next major milestone of insect history occurred around 120 million years ago, which is when most flowering plants evolved, and with it the sweet spirit of cooperation that insects and flowering plants still share to this day. In fact, some insects and flowering plants have co-evolved really neat pollination strategies so that they basically evolved together, which I think is really sweet. And as a result of insect pollination, flowering plants are now the overlords of the plant world. And thus, everything smells nicer and looks prettier. Thanks insects!

Insects are the only arthropods that have developed the ability to fly, and it has served them well. Insect's wings are just extensions of the cuticle of the thorax, so unlike birds and bats which have to sacrifice walking legs in order to evolve wings, insects are just as graceful on the ground as they are in the air. 

Metamorphosis
(10:42) But in order to be so awesome, insects had to develop this crazy thing called metamorphosis. In partial metamorphosis, the young, called nymphs, look exactly like the adult of the species, and undergo a series of molts which allows them to get bigger and bigger, but they look basically the same the whole time. Most other types of arthropods do this, and some insects, including grasshoppers and cockroaches. However, complete metamorphosis is the process unique to some insects that lets them completely change shape. Maggots turn into flies, mealworms turn into beetles, caterpillars turn into butterflies. The baby insect, called a larva- I have one right here, this is a rhinoceros beetle larva- pretty gross! hangs out and eats until it's time to build a little case around itself called a pupa, this is the rhinoceros beetle pupa, which is exceptionally creepy (creepy laugh) and then when it comes out of the pupa, it's fully grown. Rhinoceros beetle! It's like, soft at first and then it's chitin hardens up and it becomes the adult.

Crustacea
(11:39) So insects are basically wizards, but not as delicious as Crustacea, the insects of the sea! Crustaceans include crabs, lobsters, shrimp, and barnacles, and like insects, they have three body segments: head, thorax and abdomen. Although some, have a cephalothorax like spiders do. While most other arthropods have learned to love the land, very few crustaceans have. They have decided to put their energy into developing other amazing characteristics. For instance, lobsters and crayfish are like walking multitools: they have 19 pairs of appendages, some of which are claws, some are mouthparts, some are regular old walking legs. Some shrimp have evolved bioluminescence, which is pretty much the most amazing thing any animal can do as far as I can tell. And this Yeti crab... looks like a Yeti. And we've covered most of the types of animals on earth in, what? 10 minutes? Well hopefully now you can see the resemblance between these guys. I love my caterpillar.

Thank you for watching this episode of Crash Course Biology, if you want to repeat anything that we went over or do a little bit of review there's a table of contents that you can click on. Thank you to everyone who helped put this together including all of our little animal friends, and Brandon from the University of Montana for coming out and helping us out with this. And if you have any questions or ideas, please leave them below in the comments and tell me what you think we should name this one. That's all. Bye!