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Animal Kingdom, Part A
When students are taking the AP Biology test and they get tests about the different kinds of animals, it’s not uncommon for them to panic and go. "There is too many for them to be, I don’t get it." Don’t worry. The thing is, you already know a ton of the characteristics of all the animal groups. I mean look around here, there is all sorts of creatures. And you don’t seem to go, "What's that over there?" No, it’s a person. These bees here, you go I see feathers, I see beak, that’s a bird. Did you realize you just narrowed it down all the way past kingdom, past phylum, to order level?
Now looking at a bird, you didn’t need to know it’s got one ovary or hollow bones. No. You just used one or two characteristics to zero straight in on it. I’m going to help you learn how to do that for the rest of the groups in the animal kingdom. Plus, I’m going to give you this tool called the phyla genetic tree that will help evolutionary organize those characteristics, so that you’ll have an easier time memorizing it, and you’ll do better on essays on the evolution of the animal kingdom.
There is so much information about the animal kingdom, that I’m not going to be able to address n just on video. So I’m going to break it down in part A and part B. That way, also you won’t have to miss that episode of Desperate Housewives tonight.
Right now in part A, I’m going to go over what are the characteristics of the animal kingdom? Then I’ll introduce that Phyla Genetic tree I just mentioned. And last, I’ll discuss symmetry. One of the characteristics used to break apart this Phyla genetic tree into the different organisms. In part B, I’ll continue that with the difference between protostomes and deuterostomes. Then I’lladdress the evolution of the trait called a coelom. And last I’ll hit some of the key evolutionary land marks and evolution of the different kinds of organ systems.
Now to make sure that we’re all on the same page, what is it that makes an animal an animal?
Well, at the cellular level, if you take a look and you see chloroplast or cell walls, that’s not an animal. That’s probably some kind of plant. On the other hand, if you do see that there are centrioles, that’s your first clue that you’re looking at an animal. The other thing to look for, a bunch of other cells. If it is, that’s multicellular, because that’s us. Protozoan are single cell creatures.
We are heterotrophic, which means that we go after and we feed on other things. We don’t make our own food. But unlike other heterotrophs, like the fungi, we’re very fast to respond to change in our environment, using things like nerves, or muscle cells. That allows us to be able to run away from things like a fungus that might try to eat us.
The last thing that the animals all share in common is that we have two copies of every gene. And that’s called being diploid. And we also follow what’s called the gametic life cycle. That means that the only haploid cells that we ever have, are gametes or sperm in my case, or eggs which are not in my case.
Now, if you remember your characteristics of the five different kingdoms, you may remember that a lot of those characteristics that I just went through, they’re actually ones that we share mainly in common with the Protista. So if we take a look at this phyla genetic tree here, we can see that the protozoan, which are a group that are part of the Protista. They’re actually they universal ancestor of all of the animal kingdoms, even this little weird group called the Porifera, which are so bizarre that they’re given their own little subdivision of the animal kingdom called the Protozoa.
This Phyla genetic tree is used by scientists to help decide where organisms should go, in terms of their evolutionary relationship. It’s kind of like a family tree. What scientists do is they say, things down here have certain characteristics in common. And then as new adaptations arise, just like in your family, if one branch immigrated to the United States, while another one went to Botswana, that would create a division. And if you use this phyla genetic tree, when you’re studying the animal kingdom, you’ll make your life a whole lot easier because you can say well, those characteristics go down low. They’re shared by these creatures down here.
Then as we go up we start developing other characteristics, more sophisticated or complex things.
Now the big ones that I’m going to use as I go through this, will be body symmetry, which we’ll use to divide these guys of. And then I’ll go a little bit further into how the development of the gut, called Protostomes or Deuterostomes was used in dividing this branch off from that branch. Again in Part B, I’ll also go through how the evolution of the coelom led to the differentiation between these groups here, as well as these groups here.
Let’s get ready to get into the first of our groups is symmetry.
Looking at that phyla genetic tree, all those different adaptations, the easiest one to understand is body symmetry. Just take a look at your right hand and your left hand. They should look pretty similar. In fact, the right and left side of your body should be identical. That’s not just a coincidence. That’s something called bilateral symmetry. Now, why do this?
It turns out, by doing this, by using the same genes to make both sides of your body, it allows you to get away with having fewer genes. As a side note, scientists have actually figured out that one indicator of how pretty you are, is how well symmetrical your face is. If you had your right eye up here and your left eye down there, that would indicate to some people that you’ve got some problems with those genes, and you’re not as good a catch as somebody like Denzel Washington who’s got that perfect symmetry to his face.
This is the more recent condition. The more primitive condition is called radial symmetry. With radial symmetry, that’s found in groups like the Porifera, the sponges. The Porifera are so simple that some of them have even managed to master radial symmetry, as you can see with this guy. They’re just simple filter feeders and all they do is they have a bunch of cells. They’re not even properly organized into tissues. That’s different from sea anemones and jelly fish, more properly called the Cnidaria. They’ve completely mastered radial symmetry as you can see.
They’ve got multiple planes of symmetry that they can cut through their body. That’s the thing about radial symmetry.
The Cnidaria or jelly fish as I’ve mentioned before, they’ve got two layers of tissue in their bodies and a raid around the central disk is all these tentacles with stinging cells called needle ides or nemeses. Going back to the bilateral symmetry, the big thing that bilateral symmetry allowed to happen is a process known as cephalization.
Cephalization is the development of the head. This may not seem all that significant, except it’s what led to your brain. Why is that? Well if you’ve got a front end and a rear end, that allows you to recognize, hey if I’m always going this direction, let’s put all my detectors for danger and food up in the front. If you’ve got all these nerves detecting information up in the front, they start making the connections. And pretty soon you’ve got yourself some cells that are dedicated to just analyzing that data. That eventually led to the development of the simple brain.
On the other hand you can also put things like your food catchers on the front. That’s my teeth, and you’ve also got your waste disposal equipment here in the rear end.
Now, another form of specialization that you allowed, was the development of the dorsal side and ventral side. That’s our back and belly. Again, I keep talking about the specialization, that’s a mantra to be repeating when you’re writing an essay about any of this sort of stuff. Because the AP Biology people love to hear that, specialization.
You can see this in things like a shark. If you look at its ventral or belly, it’s much flatter which allows it to go along the sea floor, while it’s back got that very prominent dorsal fin.
The last group I’ll talk about is the Echinoderms, otherwise known as the star fish. They demonstrate something known as the Pentaradial symmetry. You can see that there is five lines of symmetry here. On the underside of this you can see that each of the arms has this row, that’s filled with these things called tube feet. These are weird little small sacks that they can pump full of water to extend them, and then pump some of the water out to form suction cups, so they so things like clamp on both sides of a mollusc or clam and just suck and pull it open.
To do this, they have something called a water vascular system. Now interestingly enough, the Echinoderms, while they do look like they’re radially symmetrical, they actually do have a bilateral symmetry in their juvenile or larval stage.
If we take a look at our phyla genetic tree, you can see down here, the protozoan our universal ancestor gave rise to the Porifera and Cnidaria here, which share radial symmetry. While everything else above has got bilateral symmetry. Although in the Echinoderm’s case, they only have it in their juveniles. So if you see radial symmetry, stop right there and think I’ve got it down to just three groups. If they mention that filter feeding with things called color cells, it’s the Porifera. If they mention stinging cells or two layers of tissue, that’s a Cnidarian. If they say something that pentaradial symmetry in adults, or they mention something called deuterostomes, you’re talking about the Echinodermata.
Deuterostomes you say, what’s that? I’m not going to tell you. In fact, you’re going to have to come back and watch part B of the Animal Kingdom series. In show business they call this a cliff hanger. It’s not exactly an episode of 24, but it’s the best I can do. What you do know now though is the basic characteristics of the animal kingdom. And you know all about that bilateral symmetry that we view to start breaking off that phyla genetic tree. You know that radial symmetry is held by the Cnidaria and the Porifera down at the bottom, and everything else has bilateral symmetry, except for the adult form of the Echinoderms, which is pentaradial symmetry.
In part B, I’m going to go through the development of the gut as well as the development of this thing called the Coelom. And we’ll finish off that tree.