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  • 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.

You and I both know people or dogs that we don't consider

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簡単な動物。海綿、ゼリー、タコ - クラッシュコース生物学 #22 (Simple Animals: Sponges, Jellies, & Octopuses - Crash Course Biology #22)

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    Chi-feng Liu に公開 2021 年 01 月 14 日
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