 So, let's transition now, guys, from the invertebrates to the vertebrates. As I said earlier, if it's an invertebrate, it's an animal that's lacking a backbone. So, therefore, if it's a vertebrate, it's an animal with a backbone, okay, it has a backbone. There are other vertebrate characteristics. I mentioned cephalization before. All vertebrates possess great cephalization, well-developed head area, and it also has many of these, a cranium, which is a protective skull, for the development of the brain. And of course, the vertebral column has the spinal cord. So, these guys are the most advanced in terms of nervous system development, and all these are the organ system development. And for those, we have a list of the vertebrate classes that we're going to look at. First we have class agnatha. These are the jawless fishes. So, if you think about it, that changes their diet tremendously if it's jawless versus jaws. Now, many of us don't think about it, but again, just like having the ability to use our limbs and what that affords us in terms of feeding, eating, moving, the jaws are the same thing. If you have a jaw, you're able to cross and grind and chew. If you jawless, then that changes the type of meal you're able to break down. So we have a group of fishes that are jawless, the lampreys, for example, that are jawless, and they fit into the class agnatha. And then we have the class chondrectiis, which are the cartilaginous fishes. Now, again, I talked about deep sea predators earlier in terms of squids and octopuses, but these are truly the deep sea predators in terms of the sharks and the rays. And of course, the shark is a great example because the body is made up of cartilage as opposed to bone. What does that do? It enables it to be more flexible. It can swim. The movement is much more fluid and, of course, lighter, travel at faster speeds. And therefore, again, very, very feared predatory animals in the ocean, the chondrectiis, which are the cartilaginous fishes. Then we have another group, the bony fishes, osteectiis. Of course, the bone versus cartilage, less flexibility. Still some measure of flexibility, but much more rigid. But believe it or not, they're more osteectiis bony fishes than cartilaginous fishes. So there must be much more advantage to having the bony endoskeleton than cartilage, even though in terms of the sharks and organisms like that, it does benefit them to have that flexible body plan, but also it's easier to break, I guess. Class amphibians. The amphibians, organisms like the frogs, we are very, very familiar with those like earthworms. Many of us as kids played with frogs, I'm sure. Maybe you had a pet frog. And it fits into the amphibian class. Amphibians are still tied to water for reproduction. Back in grade school, you may have learned about amphibians and reptiles. All the animals that I'm talking about, this might be the two that they stress the most. A lot of people have a hard time distinguishing an amphibian versus a reptile, because you may think, well, a salamander, and a lizard, and a crocodile, and these all look very similar. What's the difference? Well, in terms of amphibians, from a reproductive perspective, the lady eggs in water and reproduction, the development of the young, takes place in the larval stage in the water. Think of a tadpole in the case of a frog that transitions onto land. Their body skin surface is also different. It's smooth and moist, because they spend a lot of time in the water. They don't get dry, they're not drying out. But reptiles have tough, scaly skin to prevent drying out, because they spend more time on land. And still, they go back to water simply from a temperature control perspective. They just use the water to cool their body down if they're very, very hot. But from a reproductive standpoint, reptiles develop the amniotic egg. They actually lay their eggs on land, as opposed to in the water in the case of the amphibians, and therefore they are able to transition and be more connected to land and the water. And examples of reptiles, again, how about a good friend, the snake? Yes, many of us that like snakes in here, not too many. She likes snakes, wow, a dear devil. Okay, final two, aevis and mammalia. I like to call these to the most advanced, coordinated group, because when you think of aevis, the birds, they have developed the ability to fly, very, very high energy process. You think of flying, I mean, it surpasses any other process in terms of energy expenditure. And their body has adapted to that in terms of the type of endoskeleton, very light bones, and then development of feathers, which are very, very light, and enables these guys to be so successful because of the ability to fly, okay, and obviously able to, again, exist by changing environments based on, you know, the seasons, for example, if you think of the migration of birds every year, they're able to change location because of the ability to fly. They don't have to go board a plane to go there, right? They can just, they have their own inbuilt plane. And then finally the mammals, mammalia, a very interesting group because we got the egg-laying mammals, okay, the monotremes. We got the pouch mammals, the marsupials, and then we got the placental mammals where the young is in the uterus connected to the mom. So you got some diversity there in the mammalian group, but what they all have in common is that they suckle their young, they feed their young during development, the young is dependent on their parent until they're able to sustain themselves. If you think back to what I said about protostome versus deuterostome, it comes to light in the mammalian group where as you look at embryonic development, the first embryonic opening is the anus to get rid of the waste because it's being fed and then the mouse develops on the later stage. So let's recap. As we look at the animal kingdom, a lot of diversity, not everything fits these general characteristics, but for the most part, as we look at animals once again, heterotrophic, multicellular, the symmetry of the body, the type of body cavity, and then the embryonic development are some of the key things that we look at to sort of put these guys in their own group. If you look at the earlier kingdoms such as protista, there are animal-like protists. Okay, if you look at fungi, they're heterotrophic but by absorption. So they don't really fit the animal kingdom. If you look at plants and on a cellular level, animal cells lack a cell wall, they lack chloroplasts. The plants do have a cell wall, they do have chloroplasts which enable these guys to make their own foods. Animals cannot, and then in terms of acquiring food, a lot of different ways based on being a free-living animal or a parasitic animal that's literally feeding on living tissue, but heterotrophic nevertheless. And so with that, this is our introduction to the animal kingdom. We'll continue later on in terms of going into more details of characteristics associated with the animals in this kingdom.