The next major evolutionary step we mentioned was the development of a bony Endo skeleton. And we're going to see the living relatives of those early pioneers today with the bony fish. Or if you want to get that fancy jargon the name, it's ostojic thes. These organisms have an internal skeleton, mostly made of bone, which makes them much heavier than convict the ins or cartilaginous fish. And as a result, they've actually had to evolve a mechanism to keep from sinking. And they call. We call this a swim bladder. This is basically just a gas filled sack that allows a bony fish to keep them suspended at a particular depth. And believe it or not, this evolved from the lungs. Now you can see a picture of swim bladder right there, and that one obviously has been removed from fish. But you can see it right here in this internal diagram of the fish as well. So you might be wondering if these bony fish need to swim bladder in orderto prevent or to prevent themselves from sinking. What about sharks? Guess what? Sharks are actually denser than water and have to keep moving in orderto in order to stop themselves from sinking. So even though these bony fish are heavier, they actually don't have to expend as much energy in order to float. No, another rather important feature of these bony fish is called the Opara Kulum, and this is basically bony flaps that protect the gills. So what we think of as fish gills Usually when we look at a fish and we see those flaps on the outside those air actually, the thes bony structures they're not the actual gills that Gilles, they're going to be those, like, kind of fleshy internal structures. So, uh, to groups of bony fish that, Oh, we should talk about our first, the rate finfish and these guys air named for the structure of their fins. You can see it very clearly in this diagram right here. Basically, you have these parallel bones and then these webs of skin in between them. That's how the fins are created. This is actually the most diverse group of vertebrates. We said that, uh, fish were the most diverse group of vertebrates. Well, amongst all fish, these guys are the single most diverse group. And that's because actually, Thea other guys we're going to see lobe finned fish don't are not as species rich as ray finned fish, which in fact, are found in most marine and freshwater environments. They're pretty much ubiquitous and jump out of the picture here for a second. So I could just quickly point out the swim bladder right there. And here we have the gills. So again, that's like that fleshy internal structure. Where is the empirical? Um, is gonna be this bony flap covering it. All right, Moving on Loeb finfish. Uhh! The most well known example of low pond fish is an organism called a Celia can't, which was long thought to be extinct. They had found fossils of them and then, uh, within recent history living Celia camp living Celia can't was found completely overturning that assumption. And you can see a preserved Bert a preserved organism there and here you can see a drawing of one. And the reason I've included the drawing is because it's a little easier to see these defining features. These muscular lobe fins, as they're called, um, which actually is what allowed these guys to be our terrestrial vertebrate ancestors, right? Thes muscular fins that you see and let me hop out of the image here. These muscular fins that you see are what are eventually going to become limbs. And it's, you know, you can see them here on this organism, although I will say it's ah, it's a little easier to look at them and conceive of them as precursors toe limbs in the drawing. With that, let's flip the page and talk about when ah, life finally left the water for land.