Complex Animals: Annelids & Arthropods – CrashCourse Biology #23


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. Just sayin’. 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. But please, save some love
for Hexapoda, or “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! 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 their 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. 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 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 the 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. 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!

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