Hi. In this video, we'll be talking about Porifera and Cnidaria, two phyla which, despite appearances, are actually animals. Now, Porifera are more commonly known as sponges, and they're hermaphroditic organisms for the most part, meaning that they are capable of reproducing sexually. However, we also often see sponges or Porifera reproduce asexually. Now, sponges are filter feeders, meaning that they eat particles that are suspended in water, and they're going to do this by actually drawing water through their bodies. We can actually see a little model of a sponge's body right here, and they have this central cavity that's known as the spongicoel, and this opening through which the water moves is called the osculum. The actual inside of the spongicoel is lined with these cells called choanocytes, and you can see a choanocyte here; these are flagellated cells. These cells are what the sponge uses to actually feed on those suspended food particles and also bacteria. They are going to use their flagella to capture various particles, and then they're actually going to eat them by phagocytosis, a process where they take the particle into the cell and then perform intracellular digestion. So, unlike you and I who actually digest our food outside of ourselves, in our alimentary canals, these organisms will digest their food within the cell in the cytoplasm. Now, this grey barrier that kind of separates the red and yellow portions of the sponge's body is known as the mesohyl. It is basically a demarcator between the two regions of the sponge body and is basically this gelatinous matrix. It's gooey stuff. The amoebocytes are going to be very important for the mesohyl because they create these rigid structural fibers. You can almost think of it like the skeleton for the sponge, in a sense. They give the sponge body some support structures. They also are responsible for transporting nutrients, which is going to be super important for any animal.
Now, turning to Cnidarians, these are creatures like jellyfish. It's probably a little easier for you to think of them as animals as opposed to sponges, which are just so alien it's hard to even think of them as living organisms. Cnidarians are also very evolutionary mystic organisms. They are going to have a radial body plan, which means that, if we were to look down on them, we could divide them into four quadrants. This is as opposed to the bilateral body plan that we'll see later down the evolutionary line. Now, their bodies are basically just sacs for their cardiovascular cavity, which is responsible for gastro. Nutrient absorption and vascular functions, such as transporting stuff around. So, basically, it's the big 'everything' cavity. Again, these are super basic organisms; they're not going to have closed vascular systems with veins and arteries, like we do. Cnidaria come in two flavors. You can almost think of it as two body plans. You have the polyp, as you can see right here. These are polyps. This is a Hydra, actually. The polyp body plan is a cylindrical tube, which you can see right here, and attached to this cylindrical section, we have these tentacles which are used for capturing prey. Also, this segment here is responsible for adhering to feed while they're clung to a surface, but they are not sessile, meaning they don't stay sedentary forever like a barnacle or something. They can let go of whatever they are clinging to escape a predator, for example. Now, the other form of Cnidarian that we'll see is probably the form you are more familiar with. This is what jellyfish look like, and this is called a Medusa. You can see the Medusa body plan over here, and notice that it's almost like an inverted polyp. Right? Like the tentacles, instead of being on the top side, are now flipped under the creature, and so is the entryway into the gastrovascular cavity. Right? It's on the underside, whereas in the polyp, it's sort of on the top side. Now, cnidarians have these specialized cells that are used for what we tend to think of as stinging. Jellyfish like to sting their prey. Well, there's a little more going on here. These specialized stinging cells are called cnidocytes, and they contain a structure called a nematocyst. Basically, this is a penetrating barb that can be fired out of the cnidocyte. The nematocyst is technically an organelle of the cnidocyte. Its main function is to shoot out that barb, penetrate a prey for the cnidarian, and it's going to sting it, immobilize it potentially, and allow the cnidarian to then eat it. Now, these guys, you can tell, they are mushy. Right? If you've ever seen a jellyfish washed up on the beach or maybe even floating around in water, it's pretty clear that these are just gooey squishy organisms, and that's because they have a hydrostatic skeleton. Basically, their body is this fluid-filled cavity, filled with water, and surrounded by muscles. So, the pressure of the fluid in the body, that fluid pressure, actually is what gives the body its structure. It basically, the tension between the muscles and the fluid in the body supports the body almost like a skeleton. Now, the last type of cnidarians I want to briefly mention are called Anthozoa, and these are cnidarians that are like coral and sea anemones. What defines a lot of these organisms, or something that we can point to that makes them different from the guys we just talked about, is that a lot of them have exoskeletons that are made of calcium carbonate. Right? If you've ever seen a coral reef, a lot of the stuff in the coral reef has a hard structure to it. Not all of it. I mean, you know, there are sea anemones that kind of look like those dudes that flop around at a used car salesman lot. Anyways, this is just the last type of Cnidarian worth pointing out. These are going to look different from the Medusa and Hydra and other forms that we might see. Alright, with that, let's turn the page.
