Chordates - Online Tutor, Practice Problems & Exam Prep

Chordates are bilateral deuterostomes characterized by four key features: a dorsal hollow nerve cord, notochord, postanal tail, and pharyngeal gill slits. Vertebrates, a subgroup of chordates, possess vertebrae and a cranium, with evolutionary milestones including the development of jaws, bony endoskeletons, and limbs for terrestrial movement. Fish, particularly gnathostomes, exhibit diverse adaptations, including the lateral line system for detecting vibrations. Tetrapods, which evolved from lobe-finned fish, transitioned to land, showcasing adaptations like lungs and a closed circulatory system, while amphibians exemplify dual life stages in aquatic and terrestrial environments.

Chordates

Hi. In these videos, we'll be talking about chordates, and specifically, we'll be looking at vertebrates, which are a group of animals that have vertebrae. But before we get there, let’s define what a chordate is. A chordate is a bilateral deuterostome animal that has, at some point in its life cycle, 4 specific features. These features are the dorsal hollow nerve cord, the notochord, a postanal tail, and pharyngeal gill slits. The dorsal hollow nerve cord is a hollow tube that forms from the ectoderm, one of the primary germ layers, visible along the backside of the organism in this image. Below it, we see the notochord, formed from the mesoderm, another germ layer, which has a variety of functions in animals, including serving as a place for muscle attachment, crucial for movement in some cases.

The dorsal hollow nerve cord in some organisms will go on to form the central nervous system, including the brain and spinal cord. Similarly, the notochord in some chordates will eventually develop into the vertebrae. In some respects, you can think of the dorsal hollow nerve cord and the notochord as precursors to the central nervous system and the vertebrae of the spinal cord, respectively. The postanal tail, as the name suggests, is a tail that extends beyond the anus, visible here in this organism. The pharyngeal gill slits are openings between the pharynx and the external environment, also visible in the image. Interestingly, humans, for example, develop these as fetuses. While we develop, we temporarily have gill slits, which we then lose. This illustrates that these four defining features of chordates do not need to be present throughout the entire life of the chordate; they only need to appear at some point in the life cycle, often for a very brief period.

Moving on, let’s talk about two organisms that, while not vertebrates, are considered chordates: cephalochordates, such as lancelets. These organisms resemble fish in appearance, as shown in these illustrations. However, they are technically not fish; actual fish are considered more advanced. Cephalochordates use their pharyngeal gill slits for suspension feeding, and they have muscle attachments to the notochord that allow them to swim, much like fish. We also have the intriguing urochordates, such as tunicates. Tunicates, in their adult form, might not seem like chordates, as shown in this image. However, they display all necessary chordate features during their larval stage. Here, you can view a tunicate larva, showing the dorsal hollow nerve cord, the notochord, and the tail, though the gill slits are not visible in this diagram. Surprisingly, some tunicates, like the sea pineapple in Japan, are eaten and considered a delicacy, despite their flavor being likened to ammonia. They are said to taste very good when paired with sake.

Alright. With that, let's flip the page.

Vertebrates

Moving away from invertebrates, let's take a look at the vertebrates, or animals that have vertebrae and a cranium. First, let's talk about the vertebrae. Remember we said that in many cases these are going to form from the notochord, and the vertebrae are a column of segmented bone. Right? So when you look at our spine, the actual spinal cord, like the nervous tissue, is contained within the vertebrae, and you have these segmented bone structures that wrap around the spinal cord. So here in this image, we're actually looking at 3 vertebrae, 3 individual vertebrae. And looking at this snake, you can see tons of individual vertebrae in this image. I mean each single one of these projections that you see coming out of either side is attached to a vertebra or vertebrae. So there you go.

And the cranium, the other feature that defines vertebrates, is the case that encloses the brain. Right? Our noodles, noggins, whatever you want to call it. These can be made of bone or in some cases cartilage, as 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 hindbrain. Now, in the human brain, the forebrain is going to make up the most of what you see when you see a picture of a human brain, whereas these midbrain and hindbrain regions are going to be deeper structures and also structures on the brainstem, 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 are derived from a different place than the cranium. They come from these embryonic cells called neural crest cells that actually contribute to many different structures. And the reason I'm bringing up jaws, is because jaws are actually an important milestone in the evolutionary history of vertebrates. Now, early lineages of vertebrates actually had bony exoskeletons, believe it or not. They had a lot of them. 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 are very early evolutionary lineages.

A big milestone, again, is going to be the evolution of jaws and the organisms that evolved jaws, we call gnathostomes and today, we see living examples of these organisms in things like sharks or the sort of fancy term for that, chondrichthyans. But you know, these guys, just sharks basically. Now, bony endoskeleton is the next big milestone in the evolutionary history of vertebrates, and the living relatives of these organisms that first made the leap are going to be what we call ray-fin and lobe-fin fish. And we'll see examples of all of these organisms as we move through this section.

The next big milestone is limbs for moving on land. Right? The proverbial fish crawling out of the oceans as people often like to talk about it. It's a little different though. And these organisms were called tetrapods. Right? Four feet basically. And their living relatives are going to be organisms like amphibians. Now, the last major evolutionary step that we're going to talk about for vertebrates is the amniotic egg. And we see the amniotic egg today in organisms like reptiles, and we call this class of organisms amniotes.

So with that, let's flip the page and talk about gnathostomes.

Jawless Fish

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 vertebrae. We're going to look at 2 types of agnathans fish, which are also called cyclostomes, which basically means, like, circle mouth. You'll see why in a second, though much to your chagrin. And the first organism we're going to look at is the hagfish. This is maybe one of nature's grossest animals. These are scavengers that actually swim like snakes, and the reason they're so gross is see all these little dots along the hagfish? Well, they actually secrete this water-absorbing preslime material. And basically, that preslime, as it's secreted from those glands along the body, sucks up the water around the hagfish and turns into this gross mucus-like kind of slime, substance. This is useful. It allows the hagfish to, you know, escape if a predator is going after it or something, but it is also the reason that hagfish are horrifying and disgusting, as we can see right here.

Now lampreys, arguably worse than hagfish. I don't know. This is tough. Both of these guys are kind of, well, they're 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 use to feed on their hosts. They have a cartilage skeleton similar to how hagfish have cartilage vertebrae. Though it should be noted that their cartilage is actually not made from collagen. Lamprey cartilage is not made from collagen, which is one of the main components in most cartilage we see in animals.

With that, let's flip the page and turn to some more pleasant organisms.

Gnathostomes

Gnathostomes are jawed vertebrates and represent a large evolutionary step with the development of jaws. Now, gnathostomes include fish, amphibians, reptiles, and mammals. Though the early gnathostomes were basically just jawed fish. Common features among gnathostomes include jaws, larger forebrains, and paired appendages. So, if you think about us, for example, we don't just have one arm, we have two paired arms. Likewise, a lot of fish, such as the shark right here, don't have just one fin on one side, they have two fins, one on each side. Those are paired appendages.

An interesting system 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 the lateral line system has been highlighted and appears as all these white dots along the fish's base. As you can imagine, that's probably pretty useful for an organism that lives in water, especially a predator.

Living relatives of early gnathostomes are the cartilaginous fish, or chondrichthyans. That's the jargony name but, cartilaginous fish is just fine. These organisms include sharks, rays, skates, and what are called chimeras, which are these little kind of cute fish that 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 fins. Which are, again, kind of hard to see in these pictures because they are profile shots, but, you know, you get the idea. This fin right here has a pair, or a partner, that's on the other side of this fish that we can't see.

It should be noted that sharks actually use internal fertilization, and include species that are oviparous, oviparous, and viviparous. Remember that means, they lay eggs or they keep their eggs internally, and the young hatch from the eggs and leave the mother right at birth, or they give birth to live young. Right? Viviparous. If you don't remember those terms, you can go back and check out our earlier videos on animals where we covered them. Also, it should be noted that skates are oviparous, meaning they lay eggs, and rays are viviparous, meaning they give birth to live organisms. You've probably seen, if you've been to a beach before, something like this wash up on the shore. Those are actually chondrichthian eggs. You can see, this one is all dried out here in this image, but you can see in the image behind me, a nice little shark egg with the shark developing inside there.

Alright. With that, let's flip the page and talk about some other vertebrates.

Bony Fish

The next major evolutionary step we mentioned was the development of a bony endoskeleton, and we're going to see the living relatives of those early pioneers today: the bony fish, or, if you want to get that fancy jargony name, it's Actinopterygii. These organisms have an internal skeleton mostly made of bone, which makes them much heavier than chondrichthyans or cartilaginous fish. As a result, they've had to evolve a mechanism to keep from sinking called a swim bladder. This is basically just a gas-filled sack that allows bony fish to remain suspended at a particular depth, and believe it or not, this evolved from the lungs. You can see a picture of a swim bladder right there, and you, that one obviously has been removed from a 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 a swim bladder to prevent themselves from sinking, what about sharks? Guess what? Sharks are actually denser than water and have to keep moving 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 to float. Another rather important feature of these bony fish is called the operculum, and this is basically bony flaps that protect the gills. What we think of as fish gills, usually, when we look at a fish and we see those flaps on the outside, those are actually these bony structures. The gills are going to be those fleshy internal structures.

Two groups of bony fish that we should talk about are, first, the ray-finned fish. These guys are 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. Amongst all fish, these guys are the single most diverse group. And that's because the other guys we're going to see, lobe-finned fish, are not as species-rich as ray-finned fish, which are found in most marine and freshwater environments. They're pretty much ubiquitous. Let me jump out of the picture here for a second, so I can just quickly point out the swim bladder right there. And here we have the gills. So again, that's the fleshy internal structure, whereas the operculum is going to be this bony flap covering it.

Moving on to lobe-finned fish. The most well-known example of a lobe-finned fish is an organism called a coelacanth, which was long thought to be extinct. They had found fossils of them, and then, within recent history, living coelacanths were found, completely overturning that assumption. You can see a preserved, or, a preserved organism there. And here, you can see a drawing of one. 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, which actually are what allowed these guys to be our terrestrial vertebrate ancestors. These muscular fins that you see, and let me hop out of the image here, are what are eventually going to become limbs, and it's a little easier to conceive of them as precursors to limbs in the drawing.

With that, let's flip the page and talk about when life finally left the water for land.

Tetrapods

Tetrapods represent the evolutionary step of life moving on to 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, such as fingers and toes, will evolve to 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 four limbs, but they used them just to move around in the water, less in a swimming-like fashion and more in a walking-like fashion. And eventually, these organisms had to evolve a heart and a 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 what you could almost think of as an in-between species. In between those lobe fin fish and the eventual tetrapods. This organism basically, a very early tetrapod, one of those fully aquatic guys, will eventually evolve into something more akin to an early prototype of an amphibian. And eventually, these lineages give rise to an organism like this guy behind my head, 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-reptile, sort of 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 to reptiles.

So modern ancestors of the early tetrapods, as we've already said, are amphibians. And that word amphibian actually comes from Greek, it kind of means "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 ectothermic, meaning they are going to get some of their heat, their body heat, from external sources. As we said, they live in both water and land, and we're going to see their living ancestors in the forms of salamanders, frogs, and these guys you probably never heard of before called apodens, which kind of look 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, and that's what gives rise to snakes, sort of similar to what's going on here, these apodens sort of meaning like without feet, these guys lost their limbs.

So as we said, ectothermic 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 endothermic, which means that the main source of body heat is internal as we have it, and that's because our metabolism generates that heat. In fact, this is something that we've talked about previously in a video when we discussed cellular respiration because our body actually uses ATP to help generate body heat. Anyway, amphibians do have lungs, but they also can respire through their skin, which is why their skin has to be kept moist.

Amphibians generally have this metamorphosis where they go from a larval stage in the water to a terrestrial stage. Although some amphibians 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 larval stage, and then undergo a metamorphosis to the terrestrial stage. So, with frogs, probably the easiest example to point to, you know, you have those tadpoles where they look like aquatic organisms, and eventually, they lose their long tail and turn into the adult stage of a frog after undergoing metamorphosis. That's all I have for this video. See you guys next time.

Here’s what students ask on this topic:

What are the four key features that define chordates?

Chordates are defined by four key features present at some point in their life cycle: a dorsal hollow nerve cord, a notochord, a postanal tail, and pharyngeal gill slits. The dorsal hollow nerve cord forms from the ectoderm and can develop into the central nervous system, including the brain and spinal cord. The notochord, derived from the mesoderm, provides structural support and can develop into the vertebral column in vertebrates. The postanal tail extends beyond the anus and is used for locomotion in many species. Pharyngeal gill slits are openings between the pharynx and the external environment, which can function in feeding and respiration.

How do vertebrates differ from other chordates?

Vertebrates are a subgroup of chordates distinguished by the presence of a vertebral column (spine) and a cranium (skull). The vertebral column, which often forms from the notochord, is a segmented structure that encases and protects the spinal cord. The cranium encases the brain, providing protection and support. Vertebrates also exhibit more complex organ systems, including a more advanced nervous system divided into the forebrain, midbrain, and hindbrain. Additionally, vertebrates have evolved significant adaptations such as jaws, bony endoskeletons, and limbs for terrestrial movement, which are not found in other chordates like cephalochordates and urochordates.

What evolutionary milestones are significant in the history of vertebrates?

Several key evolutionary milestones mark the history of vertebrates. The development of jaws (gnathostomes) allowed for more efficient feeding and predation. The evolution of a bony endoskeleton provided structural support and facilitated movement. The transition to land involved the development of limbs (tetrapods) and the amniotic egg, which allowed for reproduction away from water. These adaptations enabled vertebrates to exploit a wide range of ecological niches. Modern vertebrates, including fish, amphibians, reptiles, and mammals, exhibit these evolutionary advancements, showcasing the diversity and adaptability of this group.

What are the differences between ray-finned and lobe-finned fish?

Ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii) differ primarily in the structure of their fins. Ray-finned fish have fins supported by long, thin bony structures called rays, which are covered by skin. This group is the most diverse among vertebrates and includes species found in various aquatic environments. Lobe-finned fish, on the other hand, have fleshy, muscular fins with a central bone structure. These fins are considered precursors to the limbs of terrestrial vertebrates. The coelacanth and lungfish are examples of lobe-finned fish, with the latter being closely related to the ancestors of tetrapods.

How do amphibians adapt to both aquatic and terrestrial environments?

Amphibians exhibit adaptations that allow them to live in both aquatic and terrestrial environments. They have moist, permeable skin that facilitates cutaneous respiration, allowing them to absorb oxygen directly from water and air. Amphibians also possess lungs for breathing air. Their life cycle typically includes an aquatic larval stage, where they breathe through gills, and a terrestrial adult stage, where they develop lungs and limbs for movement on land. This dual life strategy is reflected in their name, derived from Greek words meaning 'double life.' Amphibians are ectothermic, relying on external heat sources to regulate their body temperature.