 This gracefully swimming animal is a jellyfish. Don't be fooled by its beauty. Its sting can really hurt and for some species of jellyfish can even be deadly. The jellyfishes belong to the phylum Nideria. The name comes from Nidos, which means Nettle. Nettle is a type of plant which stings and the Niderian sting, hence the name. And where are they found? They are all aquatic and they are mostly found in oceans and seas, so they are mostly marine. They are diploblastic, that means they have two layers of tissue when they are embryos. The ectoderm that is the outer tissue layer and then there is the endoderm, the inner tissue layer. These two tissue layers cover a space inside which is the gut or their digestive tract. Now these are the first phylum, the most primitive phylum to have a tissue level of organization. This is also the first phylum to have some sort of body symmetry. In fact, if you look at this jellyfish and if you cut it with a plane in the middle, so this looks like a line, but if you imagine it three-dimensionally it'll be a plane and it cuts it into two equal halves. There can be another plane like this, another like this. Actually you can imagine infinite number of such planes which can cut the animal body into two equal halves. When there is more than one such plane, it's called radial symmetry as opposed to bilateral symmetry where there is just one plane which divides the body into two equal parts. So this is radial symmetry. Now jellyfishes have this particular shape, right? That is one of the two types of body forms found in Nideria. So the type of body form that is found in a jellyfish has a shape kind of like an umbrella. This here is a mouth and this and this, they are the tentacles which have the stings. So this type of body organization is called a medusa. Now if you take this body organization and flip it over, okay? Make it upside down so that the mouth comes up and you have something like this. So over here, this is the mouth and these are the tentacles and this body form is called a polyp. So polyp and medusa are two types of body forms which you can see in phylum Nideria. Now jellyfishes have the medusa body form. You see the mouth is at the underside and all these are the tentacles facing downwards. And what about the polyp? The examples for the poly forms are one is C anemone. So this is C anemone. Then there's Hydra. Both medusa and polyp forms have a central gastro vascular cavity. So you see this cavity over here inside the body wall over here. This is the gastro vascular cavity. Gastro because it is a place where digestion takes place. So it is the digestive tract of these animals. So they take in food through the mouth, digest it over here. And some of the food is also digested inside the cells. Some digestion happens inside the cells. So that's intracellular digestion and some digestion happens in the cavity outside the cells. So that's extracellular digestion. Both types of digestion are present in Niderians. So once the food is digested, it's absorbed and the wastes are expelled back, unfortunately, through the mouth. And so we understand why it's called gastro. Why is it called vascular? So this space, it acts not only as a digestive system of the Niderians. It also acts as the circulatory system. Vascular stands for vessels. We have blood vessels. They don't. Their circulatory system is very simple. If you recall, what our circulatory system does is it provides oxygen and nutrients to all the cells of the body and takes away wastes and carbon dioxide from all the cells of the body. This cavity does the exact same thing when food comes in through the mouth, along with it comes a lot of water because all these organisms are aquatic. So when water comes in, along with it comes oxygen. So all these body cells lining the cavity, they take in that oxygen, give out carbon dioxide in that water, and take in nutrients from the water, and give out waste products. And the wastes and carbon dioxide are expelled back into the water, surrounding water through the mouth. Hence, it is called the vascular cavity. So gastro vascular cavity because it's both the digestive tract and the circulatory system. Now let's see, do they have any other organ system? They're very simple animals, so not a lot of sophistication there. But some of the Niderians do have a skeleton. So these are corals. They in fact are a tourist attraction. They're really beautiful. They're found under the sea. And these corals, they have very hard body walls. That's because they have a skeleton made of calcium carbonate. But the most interesting feature of Niderians are the nidoblasts. So this is what they're named after, the nidoblasts. So this right here, this whole thing is a cell, which is called a nidoblast. Nidoblast is a cell which has the sting inside. And the sting itself, the structure inside, it's called the nematocyst. So the sting normally is coiled around. You see this thread-like structure? It is in a coiled formation. But when the sting is triggered by, let's say, an approaching predator or a nearby prey, then this whole thing comes out. So the sting comes out, opens up, the thread opens up, and there are these barbs over here. So what happens is this thread goes and inserts itself into the body of the prey or the predator. And the barb hurts the skin and the flesh of the predator or the prey. And when that happens, a toxin is released that gets inside the body of the prey. And sometimes it paralyzes it. Sometimes it kills it if it's a very small prey. So one of the functions of the nematocyst are offense to hunt. Another one is defense. For example, when it encounters a human being, it's intimidated by him, and hence it tries to ward him off by stinging. And then another function is attachment. Sometimes these stings, they have a glue which helps the animals stick to a surface under the ocean, so they can settle there. The stinging is not the only interesting thing about Neutarians. They also undergo alternation of generations, which is normally found in plants, right? But these animals also show a type of alternation of generations, which is also called metagenesis. In plants, the alternation of generations is between sporophytes and gametophytes, diploid and haploid types of plants. But in here, the alternation of generations is between the polyp and the medusa forms. So not all animals have both the forms. Jellyfishes have only the medusa forms. Hydra has only the polyp form. But there are some Neutarians which have both the polyp and medusa forms, which go through alternation of generations by reproduction. So here is a Neutarian, which is in the polyp form. And this here is a special type of branch, which undergoes asexual production, that is buds of the form, the medusa form. So let's say this medusa is a female. Usually the sexes are separate in Neutarians, but not necessarily. In this case, we will assume that the sexes are separate. So there is a male medusa, which has come from some other individual. So the female medusa, it will lay an egg in the water, and the male medusa will release sperms. And then fertilization happens. The zygote is formed and the zygote grows up to form the larva. The larva moves and then finds a place to settle down. So the larva gets attached to a surface under the ocean and then slowly grows up to form the polyp form. So the medisas are always swimming around in the water, whereas the polyps are usually fixed at one place. So the polyp is fixed, whereas the medisas, they swim. And when they go from the polyp to the medisa form, this is asexual reproduction, because the medisa just parted off from the poly form. And when the poly form comes from the medisa form, you saw how they went through the sperm and egg and fertilization and everything. That is true sexual reproduction. So polyp to medisa by asexual production and medisa to polyp by sexual production.