 Kaustubh was experiencing some symptoms for some time. He had abdominal pain, loss of appetite, upset stomach, nausea, weight loss, fatigue and weakness. He got tested and it turned out that he had this in his small intestine, the tapeworm. The tapeworm belongs to the phylum, platyhelminthes. Why is it called platyhelminthes? Platy means flat and helminth means worm. So these worms are flat. They are dorssoventrally flattened. What does that mean? Dorsal means the top part of the worm and ventral means the bottom part. So it's flattened from the top to bottom hence it's called dorssoventrally flattened. All the animals in this phylum are dorssoventrally flattened and hence they are called platyhelminthes. And many of them are endoparasites. Endo means inside. They are usually parasites inside the bodies of animals like the tapeworm is a parasite inside the body of a human being. Hence they are called endoparasites. There are some platyhelminths which are not parasitic at all like planaria. This is a free living animal which lives in either freshwater or seawater but since the majority of them are parasitic let's look at some of their adaptations as parasites. So this is the tapeworm and this is its head magnified. The head is magnified here and we are going to magnify the head even further. So this is the top part of the head here and these structures are the specialized structures because they are parasites. So the tapeworm they are parasites that are found in the small intestine. So what they do is they cling to the wall of the small intestine so that they are not washed off with the content of the small intestine. So they want to hold on to the wall. So how do they hold on to the wall through these structures? So you see these curved structures, these are the hooks. So hooks naturally help them to hook onto the wall of the small intestine and each of these is a sucker. So what the sucker does is it sticks to the wall and a partial vacuum is created so that helps the worm not to fall off from the wall of the small intestine. These two structures, hooks and suckers, they help the worm to adapt to their parasitic mode of living. Another interesting thing about the platyhelmets is their excretory system. So in their excretory system there is this specialized cell called the flame cell. This whole thing is the flame cell. Now it's an interesting name, a flame cell. Why? Because you see this part, right? This part are the cilia. So the cilia as you can see is shaped kind of like a flame and when the cilia beat, when they move, the flame looks like kind of like a moving flame, how a flame moves in the wind. The cilia look kind of like that, hence the cell is called the flame cell. And what the cilia do is when they beat, they draw in water and solutes inside the flame cell. And that water with solutes, it passes through this tube which has the tube cell and finally goes out into the exterior. A lot of other things happen like in the kidneys of human beings, a lot of solutes are reabsorbed. We are not going into the details of all that, you will learn more about how excretion happens in the human beings when you go to that unit. But for now just know that the flame cells they are responsible for osmoregulation. Omoregulation means regulation of the osmotic pressure which depends on the concentration of solutes in water. Basically, it means regulating the salt water balance. So if there is too much water, they excreted, if there is too much salt they don't excrete that much water, so they maintain a balance. And another thing is excretion itself. So by excretion we usually mean getting rid of nitrogenous waste products. So nitrogenous wastes are excreted by these flame cells. So this was the excretory system. Now let's look at how reproduction happens in platyhelmets. We will actually take a look at the reproduction very briefly. So every platform is a hermaphrodite. That means each individual is both male and female. So the fertilization happens inside the body of the worm. So let's say this is the egg, the sperm comes in and fuses with the egg and it happens inside the body. So the fertilization is internal. It's internal fertilization. Once the eggs are fertilized, they are shed. The animal lays the eggs outside, so these are the fertilized eggs and all these animals have one or more larval stages. So this is the larvae and there can be more than one. So one or more larval stages. Why does the animal need so many larval stages? Because many animals like the tapeworm, they go through more than one host. For example, the common tapeworm, genusoleum is a scientific name. Its intermediate host is the pig. So the life cycle of the animal makes it go through both the human body and the body of the pig and hence it needs more than one larval stages. So maybe more than one host. So after going through all that different larval stages and then more than one host, finally the larva grows up to be the adult platyhelmin, in this case the tapeworm. Some flatworms like plenaria undergoes regeneration. So if accidentally its body breaks into different parts, each of the body parts can regenerate into a full new individual like this. So the head forms, the head part forms over here. This middle part regenerates to form this individual and the tail part regenerates to this. So it broke into three parts and then each of the three parts grew up into a new individual which is what happens in regeneration.