Overview of Animals: Study with Video Lessons, Practice Problems & Examples

Animals are multicellular, heterotrophic eukaryotes that undergo complex embryonic development, starting from a zygote. They exhibit various cleavage types, such as spiral and radial cleavage, influencing their body plans. Animals can be classified as protostomes or deuterostomes based on whether the blastopore becomes the mouth or anus. Additionally, they possess distinct germ layers: ectoderm, mesoderm, and endoderm, which develop into specific tissues and organs. Symmetry types, including radial and bilateral symmetry, further define animal organization, while the nervous system, derived from the ectoderm, plays a crucial role in sensory processing and coordination.

Overview of Animals - 1

Hi. In this video, we'll be going through a general introduction to animals, and in later videos, we'll explore more specifics. So animals are multicellular, heterotrophic, eukaryotes, and we feed by ingesting our food, meaning we actually pull our food into our bodies to absorb the nutrients. Now most animals are diploid and produce gametes directly by meiosis, which is different, if you'll recall, than how, for example, plants do it, where spores are actually produced by meiosis, and their gametes are often produced by mitosis. Animals lack cell walls. Right? Plants use cellulose, fungi use chitin. We don't have that. Instead, we use an extracellular matrix for support. All animals are motile. At least at some point in every animal's life, they exhibit active movement. Right, intentional active movement. Now, most animals reproduce sexually, though there are some that reproduce asexually. But for the most part, we're going to focus on, what you could think of as, you know, more common animals. Now, animals also undergo embryonic development, and this is when the zygote is undergoing cleavage. And you actually will see more or less three patterns of birth for animals. So you have viviparous, in which the embryo will actually be nourished inside the parent, and the parent will give birth to live offspring. This is in contrast to oviparous organisms, in which the parent actually lays eggs, and the embryo is actually nourished by yolk in the egg, as opposed to the parent directly. And in ovoviviparous organisms, the eggs actually will remain inside the parent until they're ready to hatch, but the embryo is still nourished by the yolk, not directly by the parent. So humans, and mammals in general, are viviparous organisms as you might have guessed. Now it's worth noting that during embryonic development, animals actually have a lot of similarities. Here in this image, you can see the embryonic development of a fish, salamander, tortoise, chick, calf, and a human, and look at the similarities early on in development. As development goes on, organisms become more and more different, but early on, there are striking similarities. And that is because even though animals have a wide variety of different body plans and morphologies, the genes that control the development of the body are common to almost all animals, and they're called homeobox genes. And if you want to learn more about these, check out the videos on animal development. Now, animals also have tissues, and tissues are basically organized groups of similar cells that act as a functional unit. Nice example of a tissue is this right here, this is muscle tissue. Notice the, what are called striations. Basically, they're a sort of a line pattern to the tissue, and that has to do with the various filaments, the contractile filaments that allow muscle to contract. We'll learn more about that in the chapter on muscles, but for now, the important thing to take away is that all of these individual muscle cells work together as a functional unit to make up this muscle tissue. Many tissues will actually be incorporated into organs, which also act as a functional unit and are another example of how tissues can be used. The other thing that's kind of special about animals is that we have a nervous system. Now, not every animal has a nervous system, but many animals have nervous systems, especially animals that you might think are less complex or not complex enough to have a nervous system, like a worm or a jellyfish, for example. And let me jump out of the picture here for a second. As you can see, this is a worm, and this worm, believe it or not, has a brain, right there. And it has a nervous system that you can see being depicted here and here. It's this dark portion. Here, on the bottom, we're looking at a top-down look, and here, we're getting a side view of the organism's nervous system. So those are just some general features shared by animals.

Overview of Animals - 2

There's a lot of magic that happens during animal development. I mean, it's incredible to think that a single cell, this zygote, will eventually become a complex body like that of an animal. And to think that the genetics of that zygote will determine how it forms, and those genes will control whether or not it forms into an elephant or fish. I mean, it's totally crazy. Of course, it's not magic, it's biology, and it's just amazing to think about the precision control that happens during development. So let's actually take a look at animal development.

So of course, you start with the single celled zygote, right? And that's going to go through cleavage, which are a series of rapid mitotic divisions that lead to a blastula, which is a hollow ball of cells. You can see it right here. And, these blue cells are the outer ring of cells, this yellow stuff that is, representing the hollow space inside the blastula, which is called a blastocoel, in case you're curious. And, it's also worth noting that a mammalian blastula is called a blastocyst, just in case you hear that term come up.

So then this blastula is gonna go through gastrulation, which leads to the formation of a gastrula. And basically, this is the formation of the 3 germ layers, which are sort of these layers of cells that will develop into the complex, tissues and features of the animal body. So, during gastrulation, the blastula will invaginate. It will create this cavity that we call the archenteron. This cavity in here, and the opening to that cavity is called the blastopore. And this cavity will actually become the digestive tube, which is pretty crazy. What's even crazier to me is that the blastopore, that opening in the gastrula, will actually become either the mouth or the anus of the organism. So this first opening in this, what is literally just a cluster of cells, will actually lead to a major orifice of the body.

Now, these 3 germ layers, you can see here, we have the ectoderm, which is the outer layer of cells, and these are going to form nerves, organs like the or glands rather, like the adrenal medulla, the skin, the brain, the eyes, the inner ear. These are, of course, human, mammalian examples, but just to put it in context, that is easier to grasp. But the main point is that the ectoderm is the outer layer. The endoderm is the inner layer, and the endoderm is going to give rise to stuff like the lining of the digestive tract, the liver, the pancreas, and the lungs. And it's actually not depicted in this image, but I'm going to draw it in. There's also a middle layer that we call the mesoderm. And the mesoderm, again, it's that middle layer, and the mesoderm is gonna give rise to organs like the adrenal cortex, the blood, bones, the gonads, and soft tissues.

Now I said that blastopore will either turn into the anus or the mouth. And turns out that this is actually a pretty important distinction in biology, which one it becomes. Now, here, we have a nice little image of a blastula going through gastrulation, right, here's our blastopore opening up, and notice how this, cavity will actually eventually go all the way through the organism. I mean, you have an opening at 2 ends, it's a common feature of animals. Sometimes, you know, your professor might joke that, we're all just tubes, which is kind of true. In many ways, you know, our bodies are set up to just be a series of tubes. And we have, of course, tubes running through our bodies that open at one end and the other.

Now, getting back to the blastopore, depending on whether that turns into the mouth or the anus, you're either considered a protostome or deuterostome. And a protostome is an organism, like the one we see right here, that is going to have its blastopore develop into the mouth. That's what's being depicted here. Mouth and the other opening will end up being the anus. Deuterostomes are the opposite. In deuterostomes, the blastopore will actually develop into the anus, and it's the other opening that will become the mouth. You can see an example of a deuterostome right here. And you might also notice there's some other details about how the mesoderm, will form the colon, and I don't want you to worry about that too much just yet, as we haven't talked about what a colon is yet, I'm just putting that in there because technically that also applies to the definition of the protostome and deuterostome, and you can see that that's been labeled here. We have the mesoderm tissue here and here, and it is, depending on whether you're a protostome or deuterostome, doing different things to form the colon, which is a type of body cavity. It's really important to realize, given all this, that humans are actually deuterostomes, which means at one point in everyone's life, you were just nothing but an anus. Right there. Just kidding.

Alright. Let's flip the page.

Overview of Animals - 3

Hello, everyone. In this lesson, we are going to be talking about how the embryos of different types of animals develop into the entire organism. Alright. So, we've talked about protostomes, and we've talked about deuterostomes already. Protostomes and deuterostomes develop their embryos in slightly different ways. The first thing that we're going to talk about is the different types of cleavage that can occur within the embryo. Remember, cleavage is a specialized form of cell division that occurs very early in the development of the embryo. This particular type of cell division is where the cells divide, but they don't get any larger. So, the mass of the embryo is not changing, but the number of cells is. As you can see, there are many different types of cleavage, including spiral, radial, indeterminate, and determinate. We are going to have these two different types or four different types of cleavage. Now, spiral cleavage and determinate cleavage are most associated with protostomes, while radial cleavage and indeterminate cleavage are associated with deuterostomes. Spiral cleavage occurs when the plane of cell division is diagonal to the vertical axis of the embryo. This might be confusing, but essentially, the cells do not sit directly on top of one another; a cell sits in between the two bottom cells, kind of spiraling the position of the cells every time they divide.

If you were wondering what this would look like from a top-down view, in a lateral view, we have the four cells that are first formed, and then in red, these cells are positioned over the meeting area of these cells. As you can see, they are not stacked directly on top of one another but are spiraling or moving their position. This is the top view, and this is the lateral view. This is just one way that the embryo can divide its cells. Is one form of cleavage better than the other? No, there are different ways to do it. Now, you also have radial cleavage, where the plane of cell division is parallel or perpendicular to the vertical axis of the embryo. In radial cleavage, these cells sit directly on top of one another. What would that look like from the top-down view? Well, we have our four cells created from the first round of cleavage, and then, in blue, I'm going to draw these cells here, the second round of cleavage. They are going to sit directly on top of the original cells. This is the difference between radial and spiral cleavage. As mentioned before, no one form of cleavage is better than the other; it's just a different way to do it. Different types of organisms do it different ways.

Now, there's also indeterminate cleavage and determinate cleavage, which relate to the fate of these cells. Indeterminate cleavage is where the cells that arise can develop into anything in the organism. Their fate is not determined; they are not going to become liver cells or leg cells. These cells are indeterminate; they can become anything that they need to be. Determinate cleavage, on the other hand, is the exact opposite. The cells that arise are committed to differentiation. These cells have already determined their fate; they are determinately cleaved. So remember, protostomes have spiral and determinate cleavage, while deuterostomes have radial and indeterminate cleavage. Alright.

Let's scroll down and talk about the different germ layers that you can have inside of an organism. When discussing an embryo, it starts dividing its cells and organizing them in a particular way into primary germ layers. Primary germ layers are layers of cells in the embryo that are distinct and that will form very particular features of the organism. You can have diploblasts and triploblasts depending on their germ layers. Diploblasts have two germ layers, while triploblasts have three germ layers. In diploblasts, they only have an ectoderm and an endoderm in their blastula or embryo. Meanwhile, triploblasts have the three primary germ layers: endoderm, mesoderm, and ectoderm. We, as humans, are triploblastic organisms; we have three germ layers in our embryos. A representation of the embryos is shown here, with a diploblast embryo and a triploblast embryo depicted. These different germ layers will become different things: the ectoderm germ layer will become your skin, the outer layer of your body; your mesoderm will become things like your muscles and the different organs in your body, and your endoderm will create the inner lining of your body.

Now, let's talk about body cavities. Body cavities begin to form even as the embryo is forming, even in very early stages. One of the main ways we differentiate different types of animals is based on the type of body cavity they have, and you will find that this concept is highly tested upon in your lessons. The coelom is a very important structure in determining the type of animal you are looking at. The coelom is basically a body cavity inside of an animal that specifically surrounds the digestive tract and is derived from the mesoderm. This is important: the coelom is a body cavity that surrounds the digestive tract and is only made from the mesoderm. If it is a body cavity that surrounds the digestive tract made by something other than the mesoderm, then it's not the coelom; it has very specific regulations. Since this body cavity forms from the mesoderm, what kind of organism can it come from? It can only come from a triploblastic embryo or a triploblastic organism because it has to have that mesoderm. Diploblastic organisms do not have those mesoderms.

There's also something called the pseudocoelom or the fake coelom, which is a body cavity that also surrounds the digestive tract. The difference here is that the pseudocoelom forms from the mesoderm and the endoderm, whereas a true coelom only forms from the mesoderm. You can have organisms with coeloms, with pseudocoeloms, and without coeloms. An organism that has a true coelom is called a coelomate. An example depicted is an annelid, probably an earthworm, which is a true coelomate; its coelom is shown here in white. The mesoderm, in all of these examples, is red, the ectoderm is blue, and the endoderm, not shown in its color, is also depicted. The coelom is completely surrounded by red because it is only made up of the mesoderm. Now, we also have pseudocoelomates, organisms with a pseudocoelom. These are also triploblastic organisms, and their body cavity is formed from their mesoderm and their endoderm. A pseudocoelomate depicted is a nematode, a particular type of worm, referred to as a roundworm. Annelids are segmented worms, while nematodes are roundworms. You can see that the pseudocoelom is between the red and the yellow, which represents it being made from the endoderm and the mesoderm, while the true coelom is inside the mesoderm because it is only made from the mesoderm.

We also have acoelomates, organisms that lack a coelom. They lack a pseudocoelom or true coelom; they lack a body cavity. As you can see here, this flatworm lacks an internal body cavity; there is no white section at all. This is an acoelomate. I'll make sure you can see: this one is the flatworm, this one is the nematode, and this one is the annelid. These are the different types of organisms. We, as humans, are coelomates; we have a true coelom, a true body cavity. But, remember, organisms, animals, form themselves differently. Their embryos cleave differently, they form different types of germ layers, and they form different types of body cavities. Now, these forms of cleavage, determination, germ layers, and body cavities are all methods scientists and you yourselves will need to utilize to be able to identify and differentiate the various types of animals. Alright, everyone. Let's go on to our next topic.

Overview of Animals - 4

As animal bodies develop, they start to form various types of symmetry along certain axes of the body. And what we often think of as less complex organisms tend to show what's called radial symmetry, where, basically, the body parts are all arranged around one main axis. And you can see an example of radial symmetry right here in this hydra, and basically the way to think of it is that if you put a plane in any direction through the top of this hydra, it would look the same on both sides. Each side of the plane would mirror the other no matter how you shifted it. So, if we took, for example, this plane and then shifted it over to this position, we'd still have more or less the same looking halves of a hydra on either side.

Now, organisms that we typically think of when we think of animals are bilaterians, and basically, that means that they have a bilateral symmetry. This is a body plan that's divided into roughly two equal halves. So essentially, in these animals' bodies, you can draw a line through a particular point, and you will have a mirror image on either side. But you can't shift that plane around. We can't shift it this way or that way because then we'll end up with uneven halves, right? The halves won't mirror each other anymore, which is not the case for these organisms that have radial symmetry. Just to be clear, this is bilateral symmetry here. And here, this is actually an example of asymmetry. This organism doesn't show any symmetry; it's a type of sponge. And, there are organisms, some animals that don't show symmetry. However, most that we're used to thinking of are going to show bilateral symmetry. And as we'll learn, that's because there was an explosion of bilateral phyla during what's called the Cambrian explosion.

Now, there are also some important body axes to know, and that's because there's some terminology used to describe the position of things. Here we have our person, so of course, this person has bilateral symmetry. These terms are going to refer to different directions along the axes of this person's body. Anterior is things that point towards the head, that's anterior. The opposite of anterior is posterior. Which is things that point toward the tail, more or less. Of course, humans don't have tails. We still have a little tailbone there. You get the general direction. That's why sometimes your teacher might call your butt your posterior, sort of a polite way of saying it. Now, there's also ventral and dorsal, and those are going to go along, those directions are going to be perpendicular to anterior and posterior. Ventral is toward the belly, and dorsal is toward the back. And you might recall that dolphins, for example, have what's known as a dorsal fin, right? It's a fin on their backs, that's where the term comes from. So, make sure you know these terms because they will be used to describe the position of things when talking about anatomy.

Overview of Animals - 5

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.

Here’s what students ask on this topic:

What are the main characteristics that define animals?

Animals are multicellular, heterotrophic eukaryotes that ingest their food. They lack cell walls and instead have an extracellular matrix for support. Most animals are diploid and produce gametes directly by meiosis. They exhibit active movement at some point in their life cycle and undergo complex embryonic development. Animals also have tissues, which are organized groups of similar cells acting as functional units, and many possess a nervous system. Additionally, animals can reproduce sexually or asexually and have various body plans and morphologies controlled by homeobox genes.

How do protostomes and deuterostomes differ in their embryonic development?

Protostomes and deuterostomes differ primarily in the fate of the blastopore during embryonic development. In protostomes, the blastopore becomes the mouth, while in deuterostomes, it becomes the anus. Additionally, protostomes typically exhibit spiral and determinate cleavage, where cells' fates are determined early. In contrast, deuterostomes show radial and indeterminate cleavage, allowing cells to develop into any type of tissue. These differences influence the overall body plan and organization of the resulting organisms.

What are the three primary germ layers in animal embryos, and what do they develop into?

The three primary germ layers in animal embryos are the ectoderm, mesoderm, and endoderm. The ectoderm forms the outer layer and develops into the skin, brain, eyes, and nervous system. The mesoderm, the middle layer, gives rise to muscles, bones, the circulatory system, and internal organs like the kidneys and gonads. The endoderm, the innermost layer, forms the lining of the digestive tract, liver, pancreas, and lungs. These germ layers are crucial for the proper development of complex tissues and organs in animals.

What is the significance of the coelom in animal classification?

The coelom is a body cavity that surrounds the digestive tract and is derived from the mesoderm. It plays a significant role in animal classification. Animals with a true coelom, called coelomates, have a body cavity entirely surrounded by mesoderm. Pseudocoelomates have a body cavity partially lined with mesoderm and endoderm, while acoelomates lack a body cavity altogether. The presence and type of coelom influence the organization and complexity of an animal's body structure, aiding in the classification and understanding of different animal phyla.

What are the different types of body symmetry in animals?

Animals exhibit different types of body symmetry, including radial, bilateral, and asymmetry. Radial symmetry, seen in organisms like hydras, means body parts are arranged around a central axis, allowing for multiple planes of symmetry. Bilateral symmetry, common in most animals, means the body can be divided into two mirror-image halves along a single plane. Asymmetry, found in some sponges, means there is no symmetry in the body plan. These symmetry types are crucial for understanding the organization and evolutionary relationships among animals.