Name: 簡單的動物。海綿、水母和章魚--生物速成班 #22 (Simple Animals: Sponges, Jellies, & Octopuses - Crash Course Biology #22)
Uploaded: 2021-01-14T04:21:23.000Z
Duration: 11 min 31 s
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關係子句

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

We sometimes refer to them as "simple" or "Real Housewives."

But when it comes to truly simple animals,

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.

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

The more different kinds of specialized cells an animal has,

the more complex it is, and this complexity is determined

As embryos, most animals either form two layers of early tissue,

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

Let's start with the very simplest of animals,

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

More important, sponge embryos don't have any layers,

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

But these cells can transform into whatever type of cell

For this reason, some scientists argue that sponges aren't even

animals at all, they're actually colonies of cells that depend on

But for our purposes, mainly because they're multicellular,

eukaryotic organisms that can't make their own food,

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,

They got a couple of sweet evolutionary breaks that made them animals that you do NOT want

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 they form a tube that allows an animal to ingest,

This makes Cnidaria among the oldest living descendants of the

world's first diploblast, which is the common ancestor

But still, jellies and anemones and other cnidarians have only

one hole that serves as both mouth and anus,

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

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

And we can see the evidence of this breakthrough in Platyhelminthes,

the phylum of soft, unsegmented worms that includes flatworms,

Not super-handsome, but these guys are a big deal,

because they're the oldest existing phylum that is triploblastic,

So in addition to an endoderm and ectoderm,

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,

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,

and the endoderm forming the digestive and respiratory systems.

And this kind of complexity is only possible because of

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簡單的動物。海綿、水母和章魚--生物速成班 #22 (Simple Animals: Sponges, Jellies, & Octopuses - Crash Course Biology #22)

period

dumb

bacteria

reality