The nervous system is one of those really special unique things that animals have, and it actually forms from the primary germ layers. The notochord forms from the mesoderm, and this is kind of like a primitive backbone structure that, what are called chordates form. And in some animals, this will develop into the actual vertebrae of the spine, whereas in others, it's a transient structure that will go away during development. Now, the neural tube is a hollow structure that the brain and spinal cord will derive from. And that actually comes from the ectoderm folding in and creating this neural tube. You can see, the tissue for it in purple over here, and it's going to fold inward and eventually create this structure, the neural tube, which will swell in certain places, and that forms the embryonic brain, those swellings. Now, there's a trend in animals, in the evolution of animals called cephalization, which is basically a trend in which the nervous tissue becomes concentrated at the anterior end of an organism. And, you know, this is essentially how the brain comes to be. Right? This mass of neurons that integrates and processes sensory information, and is usually located at the anterior end of an organism. Now, some organisms have a central nervous system, where basically the nerves are clustered into one or more tracts that project through the body. So here, we have an example of a central nervous system. Right? We have this nerve cord, goes through the body and is, you know, a bundled pile of nerves, basically. What we think of as less complex animals tend to have, what are called nerve nets, which is basically, unlike a centralized arrangement of nerves, it's a diffuse arrangement of nerves. And it's found in radially symmetric animals. So stuff like, starfish, like in this example here. And you can see, the outline of the starfish, right, and in black, this is the nerve net of the organism. You can see that protrudes out into the little arms of the starfish. So it is not a centralized, but a diffuse arrangement of nerve cells, essentially. Another, pattern we see with animals is segmentation, which is basically just repeated body structures. So think of a worm, for example. Worms have all those little segments. In fact, there are many animals that have segments, and we can, often see this very clearly during development. So you might look at a fly and go, well, that doesn't really look segmented. But if you look at a fly during development, you can actually see those segments a little more clearly. And if you want a better idea of all this, I suggest you check out the video on development, which covers, segmentation when talking about homeobox genes, which we've also mentioned here. Now, vertebrates, speaking of segmentation, vertebrates have this, vertebral column that develops from that notochord and most are deuterostomes. And the vertebral column is segmented. Right? It's segmented into vertebrae. So even organisms like humans that don't necessarily outwardly appear segmented do have segmentation. Invertebrates lack this vertebral column, and they will still have segmented bodies. Right? Like exterior structures that are obviously segmented. Most of these are going to be protostomes. So just a little distinction to make there. And we're going to talk in much more depth about, vertebrates and invertebrates in the chapters that cover those two organisms. There's going to be a chapter on vertebrates and then another chapter on invertebrates. So get a lot more detail on those in those other chapters. Alright. I'll see you guys next time.