 This big scary animal and this tiny little thing, both of them are fish. So how are they similar and how are they different? That's what we are going to explore in this video. The great white shark belongs to the class Chondrichthys and the goldfish belongs to the class Ostichthys, both of which are fish. All the members of Chondrichthys including the great white shark, they are marine. That means they live in the seas and in the ocean. Whereas in Ostichthys you find both marine and freshwater varieties. Both the types of fish have a streamlined body. A streamlined body has this type of a shape which provides minimum resistance when something is moving through water and in air. And it's a similar shape that can be found in airplanes and in cars as well. If you look at their skeletons, the Chondrichthys have a cartilaginous skeleton. So their skeleton is made of cartilage. Chondro means cartilage and Osto means bone. So the skeleton in Ostichthys is made of bone. The fish in Chondrichthys have a mouth at the bottom. If you consider this as a top of the body and this bottom, the underside of the body, the mouth is present on the underside and that's the ventral side. So Chondrichthys have a ventral mouth. This is a ventral side while that is the dorsal side. So the mouth is on the ventral side and the Ostichthys, their mouth is neither dorsal nor ventral. It is at the end, at the very tip of their head. So they are said to have a terminal mouth. As far as gills are concerned, both are fish and hence both have gills. But in Chondrichthys, they have these things. These are slits in their skin and they are called gill slits. And if you go inside the slits, that's where the gills are present. Whereas in Ostichthys, these right here are the gills. So these are the gills. Whereas on top of the gills, there is this covering which is the operculum. So gill slits and operculum. Next we come to the scales. So the two fish have different types of scales which look different. The Chondrichthys, they have these scales which have a projection kind of like a tooth. These scales are called placoid scales. Ostichthys have several different types of scales. The commonest forms are the one on the left here is a cycloid scale. So it's circular, hence the name is cycloid. And this other scale kind of looks like the cycloid scale but they have these teeth like a comb and are called tenoid scales. Teen stands for a comb. Hence they are called tenoid scales. In Chondrichthys, the placoid scales also form teeth. So the scary teeth that you see in the mouths of sharks, those are actually nothing but modified placoid scales. Now we come to the question of how they stay afloat. Ostichthys have a swim bladder or an air bladder. So you see how the size of the swim bladder is decreasing. So the fish can control that. It can control the amount of air that is contained inside the bladder. If the amount of air is less, the fish goes towards the bottom and if there is more air, the fish floats towards the top. So how about the Chondrichthys? The Chondrichthys don't have any swim bladders or air bladders. So if they want to stay afloat or if they want to maintain their depth, then they have to keep swimming. So they have to keep swimming in order to maintain their depth. That doesn't mean that if they stop swimming, they'll die. It's just that if they don't swim, they tend to go towards the bottom of the sea. A big similarity between these two classes of fish is the presence of a two-chambered heart. So this chamber here is an atrium and this is a ventricle. So we have atria and ventricles too, but in us, there are two atria and two ventricles. Whereas in fish, there is only one of each type. Both the types of fish are poikillothermic. That means cold blooded. So neither Chondrichthys nor Ostechthys can regulate their body temperature. So we human beings, we can keep our body temperature at around 37 degrees Celsius, no matter what season or what part of the earth we are in. But the fish, they cannot do that depending on what environment they are in, their body temperature fluctuates. Hence, they're called poikillothermic or cold blooded. Coming to reproduction, their sexes are separate. That means male and female individuals are separate. And that's true for both Chondrichthys and Ostechthys. In Chondrichthys, fertilization is internal. That means the egg is there in the female body and the sperm comes in and fertilizes the egg. And these fish, the Chondrichthys, they give birth to young ones. So these are young ones. Since they give birth to young ones, they are called viviparous. And the fertilization is internal. What about Ostechthys? In Ostechthys, before fertilization itself, they lay eggs. So they lay eggs in the water. Hence, they are called oviparous. Oviparous is animals which lay eggs whereas viviparous are animals which give birth to young ones. So since the eggs are laid outside, the fertilization has to happen outside. The sperms come in and fuse with the eggs in the water itself. And hence, the fertilization is external.