 Animal form and function, one, we are discussing, and this next topic, that is about the movement and support. Before this, we are going to completely, we have gone through it, or evolutionarily, starting from the fishes, the amphibians, the reptiles, the birds, the mammals. Now, the next topic, that is about the movement and support. In this, movement and support, support, as it is said, this is something which supports the body. And the mention in this is the skeletal system. And in this too, in invertebrates, and then vertebrates. And when we talk about invertebrates, that means from porifera to ikanudameta. So, in skeletal system, in animals or in groups of animals, how does this support happen? And next, see the body. As evolution has taken place, if we see that in very primitive organisms or primitive animals, they were very small sized animals. They were macroscopic, but hardly we can see with the unneeded eye or with the naked eye. But as the evolution has progressed, this cellularization has increased, the devine of labour has increased, the multicellularization has increased, and the animal has also increased, its size has increased, its volume has increased. And obviously, when the volume has increased, the support for it is also as important. And as we will now see in different animals. So, systems involved in movement have also evolved simultaneously. That means increase in size has also happened, support has also increased, and movement has also increased. All these things have developed along with the progress of evolution. With respect to support, organisms have three kinds of skeletons. Because you must have seen, when we come across the animals, the different types of animals, this is a phyla of animals. So, in that, all the animals, they don't have the triple column, just invertebrates have an invertible column. Or there is no other bony structure. But in invertebrates, there is still support. And how is that support? One support is hydrostatic support. After that, there is exoskeleton, which is the external framework of the body. And after that, there is endoskeleton. So, in the next modules, we will read this in detail one by one. But here, hydrostatic skeleton, as an introduction, hydrostatic skeleton is that in the animal, water is pressurized. When it enters the pressure, the animal is erected. It becomes upright, it becomes a little bit, you can say, just like when we breathe in the air, when the air enters the air, it cannot support itself. But when we breathe in the air, it supports itself. And similarly, there are many animals like this, where water fills them. And that is why we call it, wherever the word hydra comes, that means water, that pertains to water. So, in this, it is the water which supports the body. And in this, we will talk about, for example, there are niderians, like sea animals, which we will talk about in the next modules. Exoskeleton, as the name suggests, it is the external framework of the body. And it is as we see in the earth reports or in other animals. And after that, endoskeleton, as the name suggests, this is the skeleton, it is the framework within the body. And inside it, there are the muscles. So, these are three kinds of skeletons, which we will talk about in detail in detail in the next modules.