moving away from invertebrates. Let's take a look at the vertebrates or animals that have vertebra and cranium. First, let's talk about the vertebra. Remember, we said that in many cases these were gonna form from the node Accord. And the vertebra are a column of segmented bone. Right. So when you look at our spine, the actual, uh, spinal cord, like the nervous tissue, is contained within the vertebra, and you have these segmented bone structures that wrap around the spinal cord. So here in this image, we're actually looking at three vertebra, three individual vertebra, and looking at this snake, you can see tons of individual vertebra in this image. I mean, each single one of thes, uh, projections that you see coming out of either side is attached. Teoh a vertebra. We're vertebrae. So there you go. And the cranium. The other feature that defines vertebrates is the case that encloses the brain, right, Our noodles and noggins, whatever you wanna call it, these could be made of bone or in some cases, cartilage is we'll see. Now it's worth noting that the brains of most vertebrates tend to be divided between three regions, which we call the forebrain, midbrain and hind brain. Now in the human brain, forebrain is going thio. Make up the most of what you see when you see a picture of a human brain, whereas thes midbrain and hind brain regions are going to be deeper structures and also structures on the brain stem. So not the prominent features of the brain. However, even basic vertebrates can have their brains divided between these regions. Now it's also worth noting that the jaws of the cranium are actually derived from a different place than the rest. I'm sorry, the jaws of the skull or derived from a different place in the cranium that come from these embryonic cells called neural crest cells that actually contribute to many different structures. And the reason I'm bringing up jaws because jaws are actually an important, uh, mile marker in the evolutionary history of vertebrates. Now, early lineages of vertebrates actually had bony exo skeletons. Believe it or not, they a lot of them had these bony exoskeleton skull kind of things. Now we don't see that in later lineages. You know, we don't see vertebrates today, for example, that have these bony exoskeletons, So those air very early evolutionary lineages. Big mile marker again is going to be the evolution of jaws And the, uh you know, these organisms that evolved jaws we call naphtha stones. And today we see, you know, living examples of these organisms in things like sharks or the sort of fancy term for that Condron Thean. But the's guys just sharks basically now bony endo skeleton is the next big mile marker in the evolutionary history of vertebrates. And the living relatives of these organisms that first made the leap are gonna be the what we call Rafe in and lobe fin fish. And we'll see examples of all of these organisms as we move through this section. The next big mile marker is limbs for moving on land, right, the proverbial, uh, fish crawling out of the oceans as people often like to talk about it. A little different, though, on these organisms were called tetrapods, right? 4 ft basically and their living relatives are going to be organisms like amphibians. Now, the last major evolutionary step that we're gonna talk about for vertebrates is the amniotic egg, and we see the amniotic egg in organisms like am neo. I'm sorry. We see the amniotic egg today in organisms like reptiles, and we call these this class of organisms. AM notes. So with that, let's flip the page and talk about Matha Stones.