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Hank introduces us to the "simplest" of the animals, complexity-wise: beginning with sponges (whose very inclusion in the list as "animals" has been called into question because they are so simple) and finishing with the most complex molluscs, octopuses and squid. We differentiate them by the number of tissue layers they have, and by the complexity of those layers.

Table of Contents:
1) Porifera 1:33
2) Cnidaria 2:36
a) Diploblasts 2:48
3) Platyhelminthes 3:33
a) Triploblasts 3:56
b) Coelom 4:36
4) Biolography 5:36
5) Nematoda 7:26
6) Rotifera 7:57
7) Molusca 8:33


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CC Kids:
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 molluscs 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. 

(1:32) 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 of 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. 

(2:36) 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. 

(2:48) 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.

(3:33) 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.

(3:56) 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 and nervous system and skin; the mesoderm forming muscles and bones and cartilage, the heart and blood and other very useful stuff; and the endoderm forming the digestive and respiratory systems.

(4:36) 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 that 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.

(5:36) 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 creative and vibrant and dangerous experience and then invite all of Kingdom Animalia to the party, like Burning Man, Comic-Con, 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. 

(7:19) 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!

(7:58) 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!

(8:33) Okay, so now for some of 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: the chitons, the 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: One, 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. Two, they also have a big, muscular foot which takes different forms in each class of mollusc. Three, they have a mantle, which in some molluscs makes a shell and in others just covers the visceral mass, And four, finally, all molluscs except bivalves have a radula, or a rasping organ on their mouths that allows them 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 mollusc might have 20,000 neurons, an octopus has half a billion. 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.