 Holy animal diversity. If you look at just this little collage of animals, first of all, I would like to have this one right here. Come live with me. I think this one is living in my bathroom at the moment. My children seem rather obsessed by this one right here. My small child is very obsessed with this one. Okay, and this one looks like he's very serious. Okay, diversity. Wow. But what do they all have in common? You know what they all have in common? This, every single one of those things, all of those guys, no matter how serious they are, they all were blastula one time. This is a single zygote that's gone through mitosis, over many times, and can't you see that it's a lovely little hollow ball of cells? That's what a blastula is. Now, think back on our plants that went through the alternation of generations, like they had that awesome life cycle, fantastic, but there was no blastula stage ever. The little zygotes that formed, whether they were haploid, diploid, whatever, they never made a hollow ball of cells called a blastula. If something makes a hollow ball of cells called a blastula, then it is an animal, every time. That's how you define an animal. Our first group of animals is going to be sponges, and I'm sorry I look at those things, I'm like, dude, this isn't an animal, but it has a blastula, so it's an animal. Now, did you stay a hollow ball of cells? Sometimes I feel like a hollow ball of cells, but the fact is I'm not a hollow ball of cells anymore. So at some point, you actually turn into a tube, a tube of hollow cells, a tube within a tube, but in order to become a tube within a tube, one hole, the other hole, you end up with a little indentation in your blastula called a blastopore. This is crazy talk. We divide organisms on our giant cladogram of animals based on weather. Their blastopore becomes a mouth or an anus. What? Okay, there it is. If the blastopore, this is the little blastopore, here's the blastula. If the blastopore becomes a mouth, then the critter is in a category called protostomes. Pro, first, stone, mouth. Protostomes got their mouth out of their blastopore. You are not a protostome. So guess what your blastopore became? Yes, your blastopore became the other hole in your tube within a tube pattern. If the blastopore becomes the anus, then you are a deuterostome, deuterostome, deuteromouth. No, thank you. But second, your mouth formed second, not first. It formed second from the other hole, not the blastopore. Did you follow that? The crazy thing is that just that fact, whether your mouth came from your blastopore or your anus came from your blastopore, and your blastopore was there when you were like, I don't know, 12 days old, like you were just a little fang. You were a hollow ball of cells that was like trying to indent. That groups us with other critters. In fact, we are more closely related to C stars and things like that. And we are related to like insects. And the logic is that if we shared this characteristic with C stars, then we probably shared a more recent common ancestor with them. We are going to start looking at the very most primitive animals and slowly build an animal cladogram. And the first group we're going to look at are a group that, well, they're cool. How are they classified as animals? No one knows, but let's look at them, they're sponges.