 This beautiful thing is a popular wedding gift in Japan. This is nothing but an animal. In fact, it belongs to the least advanced of all the animal phyla. It belongs to phylum Purifera. The animals in phylum Purifera are also called sponges. They are called Purifera because their body walls are full of pores. Why these pores are there will become apparent shortly. And because of these pores they have a spongy appearance. Hence they are called sponges. So the animals belonging to phylum Purifera are called sponges. Since they are animals they are multicellular but they are only at the cellular level of organization. That means they are so primitive that their cells are not organized into tissues like in higher animals. So they have a cellular level of organization and they are asymmetrical. Their bodies don't have any symmetry. Where can you find them? They are aquatic animals. They are found in water body mostly marine. Mostly in the oceans and the seas. So how do these animals eat? For that we need to look into the detail of their body structure. So if we take a cross section of their body this is what it looks like. So there is this big space in the middle and this is called the sponges seal. Seal stands for sealum. Sealum is a body cavity. So this is their body cavity. Hence it's called a sponges seal. And there are all these little openings that you see in their body wall. These all are called ostea. Singular is osteum and plural is ostea. And then there is this big opening at the top which is called the osculum. So what happens is they have this water transport system or the canal system because remember they're all aquatic. They're continuously submerged in water. What happens is water goes in through each of these ostea inside and then flows upwards and upwards through the osculum. So there is this constant current of water that flows in through the ostea and goes out through the osculum. So the sponges, they get their nutrients that is the food that they eat and the oxygen that they need for respiration through this water. So when the water comes in, the cells that are surrounding these pores, they take in the nutrients from that water, take in oxygen, give out carbon dioxide in the same water and give out other waste products like metabolic wastes or digestive wastes into that water and the wastes go out through the osculum in the water. So their life pretty much revolves around this water transport system or canal system. So digestion, respiration is all done through the system. Does that mean that they don't have a digestive system? Yes, that's correct. They have no organ systems. Remember, they only have cellular level of organization. No tissues, no organs, no organ system. So no digestive system either. Then how do they digest their food? So you see all these cells in the body walls. These cells, as I said, take up the nutrients from the water flowing in and digest the nutrients intracellularly. So digestion is intracellular. Digestion happens not in a digestive tract like in us, but inside the cells. So let's look at their body wall in a little bit more detail. So this is a part of their body wall. This is an osteum and water flows in like this and goes out like this. These are, each of these red cells are coano sites. So each of these is a coano site. Coano stands for collar. So they're also called collar cells. Why collar cells? You see this thing over here at the neck. It looks like a collar. And hence it's called a collar cell. And as you can see, each of these cells has a phlegelum. So these phlegela, when they beat, they produce this current that makes the water flow inside. It is the collar cells that filter nutrients from the water. Now you see these pink things in the body wall. Each of these is a spy keel. So spy keels are made either of calcium carbonate or silica, which means they're very hard substances. They give rigidity and hardness to the body walls of the sponges. So they act as an endoskeleton. What is an endoskeleton? An endoskeleton is a skeleton that gives rigidity and support to an organism. Like we have a skeleton that's also an endoskeleton. Endo because it is present inside the body. So sponges have endoskeletons too. They look different from ours, but it's a skeleton nevertheless. So in some sponges, the endoskeleton is made up of spy keels like is shown over here. In other sponges, the endoskeleton is made up of a different material. You see these fibers over here. These are spongen fibers. Spongen is just a type of protein that the sponges have and they too give rigidity to the animals and hence they to form the endoskeleton. So how do these amazing animals reproduce? So these animals can reproduce asexually. One of the way sponges reproduce asexually is fragmentation. So what happens in fragmentation is fragments start to appear. You see these things. This is where the sponges will break off. Pieces will break off and then fragments fall off and then each of these fragments can grow into a baby sponge. So these are all baby sponges and each of them can grow up to form an adult. Simple, right? Now all animals can reproduce sexually and so can sponges. So how does sexual reproduction happen? So sponges are hermaphrodites. Sponges are hermaphrodites. That means each sponge can produce both sperm and egg. However, the sperms of one sponge usually don't fertilize the eggs of the same sponge. So what happens is the sperms are released. How are they released? In the same way that other things are released through the osculum in the water curtain. So the sperms come out through the osculum and then it finds another sponge. This sponge should be the same size as the sponge, but I've shown it a little bit bigger so the things are clearer. So this sponge has this egg over here in its body and the sperms, they swim in through the osteo, just like everything else. Sperm 2 comes inside the body of a sponge through an osteum. This is an osteum and the sperm comes in and the sperm goes and fuses with the egg over there. So fertilization happens. So notice that the fertilization is happening inside the body of this animal, okay? So the sponge is not laying its egg anywhere, okay? The egg is inside the body and the sperm is coming and fusing with it. That means the fertilization is internal. Once fertilization happens, zygote is formed and the zygote, it divides by mitosis repeatedly to form the embryo. And the embryo develops, that means it grows further to form a larva. So this is a larva. So what is a larva exactly? A larva is the stage in the development of an animal which does not look like the adult. So that means the development is indirect. What do I mean by this? What I mean is that the zygote does not directly develop into the adult. There is an intermediate stage which is the larva. So development is through the larval stage. This is in contrast to human beings. In human beings what happens is the zygote grows into the embryo which grows into the fetus. Now the fetus looks very much like the adult. Very much similar to the adult, it's just much smaller in size. So in human beings the development is direct. The zygote develops to form the individual. Whereas in animals like sponges, the zygote first develops to form the larva which looks nothing like the adult form. Look over here, the larva looks like this and the adult form looks like this. They look very different. So there is a zygote, there is the larva, the intermediate form and then there is the adult. And that is why the development is called indirect. Again like everything else, how does the larva come out of the body through the osteum? It comes out through the body wall, into the canal, into the sponges seal and then it leaves the body of the animal through the osculum. And the larva hence reaches outside and then it grows up to become an adult sponge.