Chordates

hi in these videos will be talking about core dates and specifically will be looking at vertebrates, which are a group of animals that have vertebrae. But before we get there, let's actually go ahead and define what a core date is. And it is a bilateral do tourist stone animal that is going to have at some point in its life cycle for specific features. And those are the dorsal hollow nerve cord, the Noto cord, a post anal tail, and Farron Jill Gill slits. Now the dorsal hollow nerve cord is going to be a hollow tube that forms from the ectodermal, which is one of those primary germ layers, and you can see it in this image along the backside of this organism. Below it, we see the node accord, which is if formed from the miso derm, another one of those germ layers, and is going to have a variety of functions and animals, including a place for muscle attachment, which is going to be important in animal movement. In some cases, now, the dorsal hollow nerve cord, actually in some organisms, will go on to form the central nervous system that is the brain and spinal cord and likewise, the note accord in some organisms will go on to or in some coordinates, rather will go on to form the vertebra. So in in some respects, you can think of the dorsal hollow nerve coordinate accord as precursors to the central nervous system and the end the vertebra of the spinal cord, respectively. Now, the post anal tail, pretty self explanatory. Just tail that extends beyond the an issue could see it right here in this organism. And these pharyngeal gill slits thes are openings between the pharynx and the external environment. You can see them right here in the image and believe it or not, you know, humans, for example, developed these, uh, as fetuses while we develop, we actually get gills and then we lose them. So that should just reiterate the point that these four features that define coordinates are not present during the entire life of the court date. They just have to be there at some point in the life cycle. And that could be very brief Window indeed. Moving on. Let's talk about two organisms that, while are not vertebrates, are considered to be coordinates. And these are Cephalon coordinates, which are organisms like lance slits and the's are kind of like fish, in a sense, I mean, you can see a picture of one there. Uh, here is a more anatomical diagram of it. They kind of look like fish, though they are technically not fish. Uh, in fact, fish will evolve. Uh, you know, later, down the line these air very primitive organisms, and they use their pharyngeal gilles slits. Thio feed in sort of suspension feeding process. And they will have muscle attachments to that note accord that allow them to swim. And again, they're going to swim. Kind of like fish they, you know, have that sort of appearance of being fish, even though again, they're not fish. Now. We also have these quite strange organisms called your accord dates. And a new example of your court date is an organism called a tunic. It and these guys, they look kind of like this later in life. You're probably going How is that a court date? Well, they actually Onley get these coordinate features as larva. So here you can see a tunic, it larva, and you can see all the necessary features of the coordinate. Let me jump out of the image and highlight those for you. We have the nerve cord. That's gonna be that dorsal hollow nerve cord. We have a noto cord. We have. We have the tail right here, and we can't actually see it on this diagram. But these guys are gonna have some gill slits. All right, so, uh, believe they're not some tunic, its air actually eaten, in fact, considered a delicacy. For example, sea pineapples in Japan, despite having a flavor that's described to be similar to that of ammonia cleaning chemical. Apparently, it tastes very good when paired with sake. I don't know. Go figure anyhow, that let's flip the page.

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.

fish are by far the most diverse group of vertebrates with more species than all other groups of vertebrates combined. Now we said that the evolution of Jaws was a very important step. But before we had jawed fish, we had jawless fish and jawless fish actually lack a backbone, but they have cartilage vertebrate. Uh, we're gonna look at two types of jawless fish which are also called cyclist stones, which basically means like circle mouth. You'll see wine second, though much to your chagrin. And the first organism we're gonna look at is the hag fish. This is maybe one of nature's grossest animals. Uh, these air scavengers that actually swim like snakes and the reason they're so gross is you see all these little dots along the hag fish? Well, they actually secrete this water absorbing pre slime material. And basically that priest lime, as it's secreted from those glands along the body, sucks up the water around the hag fish and turns into this gross mucus e kind of slime a substance. This is useful. It allows the hag fish to, you know, escape if it's ah, predators going after their something. But it is also the reason that hag Fisher, horrifying and disgusting like we can see right here now lampreys arguably worse than hag fish. I don't know thistles tough. Both these guys air kind of. Well, there is no one's favorite animal. Let me put it that way. Lampreys are parasites with that characteristic circular mouth, except theirs is filled with these horrible teeth that they used to feed on their hosts. They have a cartilage skeleton, similar to have how hag fish have cartilage vertebra, though it should be noted that their cartilage is actually not made from college. In Lane, Priest cartilage is not made from collagen, which is one of the main component components in most cartilage we see in animals. With that, let's flip the page and turn to some more pleasant organisms.

naphtha stones are jawed vertebrates and represent that large evolutionary step with the of the development of jaws. Now, naphtha stones include fish, amphibians, reptiles and mammals. Though the early Matha stones were basically just jawed fish. Now common features amongst Matha stones include jaws, larger four brains and paired appendages. So if you think about us, for example, we don't just have one arm. We have to paired arms. Likewise, for example, you know Ah, lot of fish. You see this shark right here? They don't have just one fin on one side. They have to Finn's one on each side, those air paired appendages. Now, an interesting thing that we'll see in some of these organisms is called the lateral line system, which is a sensory system that detects movements and vibrations in the water. You can see it highlighted here in this shark in red, and you can also see a real photograph of a fish here where it has, uh, the lateral line system has been highlighted and, it appears, is all these white dots along the fishes face here. Now, as you could imagine, that's probably pretty useful for an organism that lives in water especially a predator. So living relatives of early Nath systems are the cartilaginous fish or Kendrick The INS. That's the jargon. E name cartilaginous Fish is just fine. These are organisms like sharks raise skates and what are called Kim eras, which are thes little kind of cute fish. You see right here. The defining feature for these organisms is that they have a cartilaginous skeleton, meaning a skeleton made of cartilage. And they have those paired Finns, which are again kind of hard to see in these pictures because there are profile shots. But, you know, you got the idea. This Finn right here has a pair or a partner, rather, that's on the other side of this fish that we can't see. So it should be noted that sharks actually use internal fertilization and includes species that air over Paris, vivid Paris and vivid Paris. Remember, that means, uh, they lay eggs or they keep their eggs internally and, uh, the young hatch from the eggs and leave the mother right at birth. Or they give birth to live young, right, vivid Paris. If you don't remember those terms, could go back and check out our earlier videos on animals where we covered them. Uh, also should be noticed that skates are over Paris, meaning they lay eggs and raise our vivid Paris meaning they give birth thio live organisms. And you've probably seen if you've been to a beach before, you might have seen something like this washed up on the shore. Those are actually uhh could Rick Thean eggs. And you can see this is this one is all dried out here in this image, but you can see in the image behind me Nice little shark egg with the shark developing inside there. All right with that, let's actually flip the page and talk about some other vertebrates.

The next major evolutionary step we mentioned was the development of a bony Endo skeleton. And we're going to see the living relatives of those early pioneers today with the bony fish. Or if you want to get that fancy jargon the name, it's ostojic thes. These organisms have an internal skeleton, mostly made of bone, which makes them much heavier than convict the ins or cartilaginous fish. And as a result, they've actually had to evolve a mechanism to keep from sinking. And they call. We call this a swim bladder. This is basically just a gas filled sack that allows a bony fish to keep them suspended at a particular depth. And believe it or not, this evolved from the lungs. Now you can see a picture of swim bladder right there, and that one obviously has been removed from fish. But you can see it right here in this internal diagram of the fish as well. So you might be wondering if these bony fish need to swim bladder in orderto prevent or to prevent themselves from sinking. What about sharks? Guess what? Sharks are actually denser than water and have to keep moving in orderto in order to stop themselves from sinking. So even though these bony fish are heavier, they actually don't have to expend as much energy in order to float. No, another rather important feature of these bony fish is called the Opara Kulum, and this is basically bony flaps that protect the gills. So what we think of as fish gills Usually when we look at a fish and we see those flaps on the outside those air actually, the thes bony structures they're not the actual gills that Gilles, they're going to be those, like, kind of fleshy internal structures. So, uh, to groups of bony fish that, Oh, we should talk about our first, the rate finfish and these guys air named for the structure of their fins. You can see it very clearly in this diagram right here. Basically, you have these parallel bones and then these webs of skin in between them. That's how the fins are created. This is actually the most diverse group of vertebrates. We said that, uh, fish were the most diverse group of vertebrates. Well, amongst all fish, these guys are the single most diverse group. And that's because actually, Thea other guys we're going to see lobe finned fish don't are not as species rich as ray finned fish, which in fact, are found in most marine and freshwater environments. They're pretty much ubiquitous and jump out of the picture here for a second. So I could just quickly point out the swim bladder right there. And here we have the gills. So again, that's like that fleshy internal structure. Where is the empirical? Um, is gonna be this bony flap covering it. All right, Moving on Loeb finfish. Uhh! The most well known example of low pond fish is an organism called a Celia can't, which was long thought to be extinct. They had found fossils of them and then, uh, within recent history living Celia camp living Celia can't was found completely overturning that assumption. And you can see a preserved Bert a preserved organism there and here you can see a drawing of one. And the reason I've included the drawing is because it's a little easier to see these defining features. These muscular lobe fins, as they're called, um, which actually is what allowed these guys to be our terrestrial vertebrate ancestors, right? Thes muscular fins that you see and let me hop out of the image here. These muscular fins that you see are what are eventually going to become limbs. And it's, you know, you can see them here on this organism, although I will say it's ah, it's a little easier to look at them and conceive of them as precursors toe limbs in the drawing. With that, let's flip the page and talk about when ah, life finally left the water for land.

tetrapods represent the evolutionary step of life moving on tow land, and these four limbed vertebrates include amphibians, reptiles and mammals like us. Now these limbs, as we said, are derived from those muscular fish fins to support the organisms. Weight on land and eventually digits will be evolved to help. As digits is in fingers, toes, that sort of thing will be evolved. Thio help efficiently transfer muscle force. It's a common misconception that all of a sudden one of these fish just crawled up on land. Early tetrapods were actually fully aquatic, so they had these four limbs. But they used them just to move around in the water, less in a swimming like fashion and Maurin a walking like fashion. And eventually, uh, these organisms had thio evolve heart and circulatory system in order to deliver sufficient oxygen to their limb muscles while they lived on land. So we're going to see one of the important features of these organisms is the closed circulatory system. Now, here in this image, you can kind of see the evolution of sort of this. You could almost think of this as an in between species in between those lobe fin fish and the, you know, eventual tetrapods. This organism basically a very early tetrapod, one of those fully aquatic guys who will eventually evolved into something more akin to this guy, which is going to be, you know, sort of like your amphibian. You know, early early, uh, sort of prototype of an amphibian. And eventually we're going to see these or these, uh, lineages give rise to a new organism like this guy behind my bed in my head. Rather not my bed. God, I would be terrified if this guy was behind my bed. Never sleep in there again. But this organism, which is kind of going to be like a proto reptiles, sort of, uh, you know, like, this guy was the in between on the way to the tetrapod. This guy here behind my head, not my bed is going to be that in between, on the way Thio reptiles. So modern ancestors of the early tetrapods, as we've already said, our M for Vivian's and that word amphibian actually comes from Greek. It kind of means a double life. You could think of it as, and that's because these organisms are going to live both in water and on land. Now these organisms are ecto thermic, meaning they are going thio get some of their heat, their body heat from external sources, Azzawi said. They live in both water and land, and we're going to see, uh, they're living ancestors in the forms of salamanders, frogs and these guys you probably never heard of before, called ape Oden's, which kind of looked like worms. Believe it or not, they are amphibians. It's kind of like a snake situation where you had reptiles with limbs and then some of them lost their limbs right. And that's what gives rise Thio snakes sort of similar to what's going on here. These ape Oden's ape Oden sort of meaning like without feet. These guys lost their limbs. So as we said, extra thermic means that the main source of body heat is external to the organism and those organisms absorb it. This is in contrast to the term endo thermic, which means that the main source of body heat is internal, like we have it, and that's because our metabolism generates that heat. In fact, this is something that we've talked about, uh, previously in a video when we discussed cellular respiration because our body actually uses a teepee to help generate body heat. Anyways, uh, amphibians do have lungs, but they also can respond iron through their skin, which is why their skin has to be kept moist. Um, and amphibians uhh. Also generally have this metamorphosis where they go from a larval stage in the water to a terrestrial stage. Although some amphibians, uh, live almost entirely on land and some live almost entirely in water. So there's a lot of variation. But the important trend is that most of them will lay their eggs in the water, have this sort of larval stage and then undergo a metamorphosis to the terrestrial stage. See, uh, with frogs. Probably the easiest example. 2.2. You know, you have those tadpoles. Where there they look like aquatic organisms. And eventually they lose their long tail and turn into the adult stage of frogs after undergoing metamorphosis. That's all I have for this video. I'll see you guys next time