Simple Animals: Sponges, Jellies, & Octopuses – Crash Course Biology #22


You and I both know people or
dogs that we don’t consider particularly sophisticated. We sometimes refer to them as
“simple” or “Real Housewives.” But when it comes to
truly simple animals, we shouldn’t underestimate them. Because the animal phyla that we
describe as being the least complex actually offer us a vivid
way of understanding how animals are structured, and
also how they evolved. Simple doesn’t always mean dumb. Unlike those dullards that
we’ve all met in our lives, animals aren’t considered simple
because they apparently take things for “granite” or they think
that reality TV is…reality. Their simplicity has to do
with their tissue complexity. As you know, almost all animals’
cells are organized into tissues that perform specialized functions. The more different kinds of
specialized cells an animal has, the more complex it is,
and this complexity is determined in the embryonic phase. As embryos, most animals either
form two layers of early tissue, called germ layers,
or they form three. By exploring the very simplest
phyla, from animals with no layers at all, aka sponges, to the
most basic of three-layer animals, like mollusks you can see how a
not-totally-amazing-sounding change in tissue results in truly
fundamental and amazing changes. So the places in the animal family
tree where these transitions take place, from no
layers to two layers, and from two layers to three,
are some of the most important benchmarks in animal evolution. Let’s start with the
very simplest of animals, in the phylum Porifera:
the sponges! They diverged from protists
probably 600 million years ago and not a whole lot has changed
for them since then. If you’ve been paying attention,
you’ve noticed by now that almost nothing that applies to other
animals applies to sponges. That’s because they’re
so freaking simple. They can’t move; they just hang
out and filter water for food like bacteria, while some host
photosynthesizing microbes and mooch off them. More important, sponge
embryos don’t have any layers, they just have cells. This means that sponges don’t have
specialized tissues or organs. And their cells can
take different forms. Some have flagella to
force water into the sponge; some are more amoeba-like
and wander around distributing nutrients. But these cells can transform
into whatever type of cell the sponge needs. For this reason, some scientists
argue that sponges aren’t even animals at all, they’re actually
colonies of cells that depend on each other to function. But for our purposes, mainly
because they’re multicellular, eukaryotic organisms that
can’t make their own food, they still count. And they’ve managed to
diversify into nearly 10,000 different species,
so good for them. Things get more interesting with
Cnidaria, which include jellies, sea anemones, corals, and hydras. They got a couple of sweet evolutionary breaks
that made them animals that you do NOT want to mess with. The first and most important
break is that they develop two germ layers.
You’ll remember these layers are called the endoderm,
or the “inside” -derm, and the ectoderm,
the “outside” -derm, and they form a tube that
allows an animal to ingest, digest and get rid of stuff. This makes Cnidaria among the
oldest living descendants of the world’s first diploblast,
which is the common ancestor of all “true animals.” But still, jellies and anemones
and other cnidarians have only one hole that serves
as both mouth and anus, and they don’t have any organs. So, still pretty simple. Their second evolutionary
break is in their ectoderm, which contains stinging
cells called cnidocysts. Think Portuguese Man o’ War,
I once stepped on a dead one, it was dead, LONG dead,
and I wanted someone to cut my foot off it hurt so much. So now we’ve got two-layer
animals swimming around, able to move and eat and
poop and defend themselves. The animal kingdom is just one
evolutionary breakthrough away from a huge, like, explosion! And we can see the evidence of this
breakthrough in Platyhelminthes, the phylum of soft, unsegmented
worms that includes flatworms, planaria, tapeworms, and flukes. Not super-handsome, but
these guys are a big deal, because they’re the oldest existing
phylum that is triploblastic, or has three germ layers. So in addition to an
endoderm and ectoderm, their embryos form a mesoderm. I know it sounds like just
another piece of toast and turkey on a club sandwich, but this
development changes everything. Platyhelminthes are
themselves pretty simple, but a couple of
phyla up the ranks, this new layer allows animals
to form true organ systems: the ectoderm forming the brain,
nervous system and skin; the mesoderm forming muscles,
bones, cartilage, the heart, blood and other very useful stuff; and the endoderm forming the
digestive and respiratory systems. And this kind of complexity
is only possible because of one of the mesoderm’s
key features, the coelom, a fluid-filled cavity that stores
and protects the major organs. It allows the internal organs to
move independent of the body wall, and the fluid can provide
some shock resistance. Coeloms are where all the
action happens when it comes to organ systems, but not
all triploblasts have them. From here on, we can assess
the complexity of an animal by whether it has a coelom or not,
and if so, how complete it is. For instance, because they’re
the simplest of the triploblasts, Platyhelminthes have their
mouths and buttholes on opposite ends of their bodies,
which is awesome for them! But they’re acoelomates,
they don’t have a coelom, which tells us
they’re still on the shallow end of the
pool, complexity-wise. To give you an idea of
how simple, you can cut a Platyhelminthes in
half, and both of the pieces will happily continue
on with their wormy business. That, my friends, is simplicity. Now, you probably haven’t
forgotten that I mentioned an explosion a minute ago. Well, I’m not going
to taunt you with talk of explosions
without giving you one. [BIOLO-GRAPHY] The Cambrian Explosion! Not long after germ layers became
a thing, say 535 million years ago, life on earth was undergoing
some pretty terrific and rapid innovations. Over about 10 or 12
million years, about half of the animal phyla that
exist today started to appear. It remains the most biologically
productive period in history. Think of the most exciting,
vibrant, creative, dangerous experience and then
invite all of Kingdom Animalia to the party. Like Burning Man, ComicCon,
and Coachella all at once. This is when animals
started to look and behave as we know them today. Before the Cambrian,
most of the big animals were slow and soft-bodied
and ate algae or scavenged. But this explosion of
diversity brought all kinds of new adaptations,
including predatory ones, like claws, and defensive ones
like spikes and armored plates. Shells and mineral skeletons
made their first appearances. In fact, the adaptations were
so many and so abrupt that in the 1800s the abundance
of fossils from this period was used to argue
against evolution. Scientists offer a lot
of different theories about what caused this explosion. It was probably a combination
of a few of these things. For one, oxygen levels became
very high in Cambrian seas, which allowed for larger
bodies and higher metabolisms. It’s also thought that
ocean chemistry changed, with more minerals becoming
available for the production of shells and skeletons. And of course, with more
diversity comes more competition and predation, which
drove selective pressures on animals to become
either better at hunting or better at defending themselves. It’s pretty near the top of
my list of places I want to go once I put the finishing
touches on my time machine. But for now, we still have
many modern animal phyla to remind us of this
time of crazy awesomeness. So flukes are cool and
all, but things start to get more complex with another
phylum of mostly nasty parasites, Nematoda, unsegmented roundworms. These guys are pseudocoelomates,
meaning they have an incomplete body cavity. Unlike a true coelomate,
whose body cavity is contained within the mesoderm, pseudocoelomates
sort of improvise one between the mesoderm
and the endoderm. The vast majority of
nematodes live in soil, where they eat bacteria or
fungus or parasitize plant roots. But humans host at least
50 nematode species, including hookworms, which burrow
into our intestines and treat us like some kind of food court. But most nematodes
are very very small: a single teaspoon of forest soil
can have several hundred in it! Rotifera, meanwhile, are
tiny filter-feeding animals that live mostly in fresh or
salt water, though some of them can live in damp soil. They’re also pseudocoelomates
like nematodes, and although they are way smaller
than most flatworms, a big honkin’ rotifer is
like 2 millimeters long, they’re anatomically more
complex, as they have a stomach, jaws and a little tiny anus. My favorite fun fact about
Rotifera is that many of its species are known
to exist entirely of females, and they reproduce through
unfertilized eggs. Fossils of rotifers have been
found as old as 35 million years, and in many cases, there’s
not a dude to be found. You go girls! Okay, so now for the big
dogs: the phylum Mollusca. Molluscs might be kind of
simple, but they’re amazing and some of them
are incredibly smart. They take four
different basic forms: chitons, snails, bivalves,
and octopi and squid. Now, I realize it can be
hard to see how an oyster and an octopus might be
related, but molluscs have some important similarities: They all have a visceral
mass, which is a true coelom a body cavity completely
within the mesoderm that contains most of
the internal organs. They also have a big, muscular
foot which takes different forms in each class of mollusc. They have a mantle, which
in some molluscs makes a shell and in others
just covers the visceral mass, And finally, all molluscs
except bivalves have a radula, or a rasping organ on their mouths
that they use to scrape up food. So, chitons, are these
headless marine animals, covered with a plated shell
on one side, and they use their foot to move around
on rocks, scraping off algae with their radula. You know about bivalves.
They have shells that are divided into two hinged
halves, like clams and scallops. They’re filter feeders, so
they trap particles of food in the mucus that
covers their gills. Snails and slugs are gastropods. One thing that sets them
apart is a process called torsion, in which the visceral mass
twists to the side during embryonic development, so
that by the end of it, its anus is basically
right above it’s head. Most gastropods also have
a single, spiraled shell and most use their radula to
graze on algae and plants. And last, but certainly not
least, we have the cephalopods, which are the kings of the
Molluscs, as far as I’m concerned. Cephalopods include octopi
and squid, and they are obviously a lot different from
other molluscs. For starters, they have tentacles
that they use to grab their prey, which they then bite
with their beaks and immobilize with
poisonous saliva. And the foot of a
cephalopod has been modified into a really powerful
muscle that shoots out water to help it move and
steer through the water. But probably the coolest
thing about cephalopods is how smart they are. While a typical mollusk
might have 20,000 neurons, an octopus has
half a billion neurons. If you just do a YouTube
search for octopus, you’ll find all kinds of
videos of them opening jars and stealing peoples’
video cameras. They’re like freaking
ocean ninjas. Cephalopods got skillz. So remember, simple doesn’t
equal dumb, there’s a lot to learn from our
less-developed cousins. Next time we’ll talk about
even more complex animals and what we have
to learn from them. Until then: Thank you for watching
Crash Course Biology, If you want to review anything
that we discussed in this video we’ve put a table of contents
over on one of my sides, I can never remember
which one it is. I think it’s THAT side, yeah THAT side? I’m getting a nod. If you have questions on simple
animals, or other topics, you can get in touch with
us on Facebook or Twitter, or of course, down in
the comments below. Goodbye.

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