AP Biology Vertebrates

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We are going to continue our discussion of animals with the vertebrates.0002

Vertebrates are all members of the phylum Chordata.0008

However, as I mentioned at the end of the last lecture, some members of Chordata are invertebrates, so this phylum is mixed.0012

It contains both invertebrates and vertebrates, so we will be covering a few invertebrates at the beginning of the lesson today.0021

The chordates are a group that includes a huge diversity of organisms.0029

Chordates live in fresh water and marine environments. They also live on land.0034

Some walk. Others swim, and still, others fly.0038

Chordates are deuterostomes, and they have a true coelom, so they are coelomates; and they are bilaterally symmetrical.0043

Chordates are united by the four characteristics that they share, and these are characteristics that they may not possess as adults.0057

But at some point in a chordates development, it possesses these four characteristics.0065

The first one is a notochord. Also, they have a dorsal hollow nerve chord, pharyngeal clefts and arches and a post-anal tail.0071

Starting with a notochord which means back string, a notochord is a rod-shaped flexible structure, and it is located dorsal to the GI tube.0084

So, the notochord is right here. This blue tube represents the GI tube.0097

Now, some terminology: dorsal means above, and ventral means below.0106

The notochord here is dorsal to the GI tube, but it is ventral or below the nerve chord.0113

Here, we have the notochord and just above it, the nerve chord.0123

Now, as I mentioned, this is a flexible rod-type structure, and be careful to not mixed it up with an actual backbone or a vertebral column, it is not.0130

In chordates that are invertebrates, those that lack a backbone, this structure, the notochord, can fulfil the functions of providing support, for example.0142

But most adult chordates lack a notochord.0152

Most chordates are vertebrates, and along the way in development, the notochord disappears.0156

And it is replaced by the backbone, by the vertebral column, and there are only vestiges of the notochord that remain in most adult chordates.0162

The notochord is composed of large cells that are surrounded by a strong fibrous tissue.0172

OK, that is the first structure that all chordates possess at some point in their development.0178

The second structure is a dorsal hollow nerve chord, so dorsal, it is above the notochord.0183

And we have talked about nerve chords before, but one thing that makes this type of nerve chord different is the fact that it is hollow.0192

The nerve chord differentiates into the central nervous system, and we will talk about this system later on when we talk about animal physiology.0201

But, the CNS right now, just what you just need to know is that the CNS consists of the brain and spinal cord.0211

The third structure is pharyngeal clefts and arches.0222

These slits right here represent the pharyngeal clefts and arches, and these are located in the pharynx, which is located just behind the mouth.0227

Visceral arches begin as pouches, so they begin as pairs of pouches in the pharynx with grooves or clefts between them.0241

We have a series of pouches with grooves between them, and eventually, these grooves or clefts develop so they open up to the outside of the body.0251

The result is that there are these slits that allow communication between the outside of an organism's body and the pharynx on the inside.0263

That is at some point in development in the embryo as the organism is developing.0272

But eventually, these arches and clefts develop into different structures depending on the species of animal and its life style.0278

For example, if you look at filter feeders like some of the simpler chordates,0286

the pharyngeal clefts and arches will develop into structures that the animal can use to string water and catch food.0291

For filter feeders, these slits might develop into the filtration device.0298

For aquatic chordates such as fish, these structures can develop into gills. They could develop into a filtration device, so I will just put filter.0304

They could develop into gills, and remember that for aquatic animals, gills allow for gas exchange.0314

In tetrapods, we are going to talk about tetrapods.0323

But, tetrapods are animals that either have four limbs or descended from an ancestral animal that had four limbs for example mammals, birds and reptiles.0325

In tetrapods, the pharyngeal clefts differentiate into parts of the ear such as the Eustachian tubes.0337

So, I will just put "part of the ear" and some glands that are located in the neck, glands in the neck.0344

The pharyngeal clefts and arches differentiate along various pathways depending on the species of organism.0353

That is the third structure that all chordates possess at some point.0360

Finally, all chordates possess a post-anal tail, a tail that is located posterior to the anus, and some species retain this tail into adulthood.0365

And it can even be very muscular and help the animal with balance on land or swimming in the water.0376

Many species simply have a tail during embryological development, and they lose it later on.0385

In humans, the tail is lost. It is reabsorbed, but humans have a vestige of this tail and that is the coccyx or tailbone.0390

This is an overview of the phylum Chordata. Now, we are going to focus in on the invertebrate members of this phylum.0402

Chordates originated during the Cambrian period over 500 million years ago.0411

And the two invertebrate groups that we are going to start with are members of the subphylum Cephalochordata. That is the lancelets.0416

And we are going to talk about subphylum Urochordata. These are the tunicates also known as sea squirts.0425

And then, in a few minutes will cover the hagfishes.0432

Starting out with the lancelets. They get their name from their shape.0434

They are shaped like a blade or a lance, and lancelets, again, are invertebrate chordates. They are bottom dwellers.0438

So, they are bottom dwellers. They live at the bottom of the ocean.0447

They are also filter feeders.0452

They are small. They are just a few centimeters long, and they burrow into the sands.0458

Lancelets retain the chordate structures that I mentioned into adulthood.0465

So, even as adults, lancelets have a notochord, a tail, pharyngeal slits and a nerve chord.0470

They need the notochord for support since they are invertebrates, and they lack a backbone. They lack bones.0477

The second group of invertebrate chordates are the tunicates or sea squirts, and as larva, sea squirts can swim; and they have tails.0484

So, the larva have, tails, and they swim. However, the adults are sedentary filter feeders.0500

And morphologically, the adults are quite different than the larva.0514

Adult tunicates have a siphon.0519

And this is how they got their name sea squirts because they draw water in one siphon and then, squirt water out the outgoing siphon.0521

The third group of invertebrate chordates are the hagfishes.0536

The hag fishes are jawless marine animals, and they are scavengers. They have a notochord that they retain into adulthood.0544

Hagfishes are shaped like snakes. They look, kind of, like snakes, although, they are not snakes; and they live on the floor of the ocean.0559

Now, the difference between this group and the two groups that I just talked about is that they belong to a taxonomic group called the craniates.0565

All of the vertebrates are craniates.0574

Remember, we are taking about phylum Chordata. We are talking about invertebrate chordates, and then, we are going to talk about vertebrates.0579

All of the vertebrates are craniates as are the hagfishes, and as the name suggests, they have a head end.0587

We talked earlier about cephalization, for example, with the invertebrates. We have discussed that molluscs have cephalization.0595

There is a head end where sensory organs are concentrated, and hagfishes have cephalization, and they are craniates.0603

There are other members of Chordata that are now extinct that I could discuss.0613

But, we are going to focus today on those chordates that are still in existence, so I am focussing on these groups along with the vertebrates.0621

Those are the three groups of invertebrate chordates that you should be familiar with.0633

And now, we are going to spend the majority of time on the vertebrates- the focus of today's lesson.0638

Vertebrates have a backbone or a vertebral column, and this is a structure that is strong yet, flexible.0644

It allows for movement but at the same time protects the spinal cord that it is surrounds.0654

The first vertebrates were jawless fish.0662

We talked about hagfishes, which are jawless fish, but they are invertebrates. Well, the early vertebrates were also jawless fish.0665

There are group of jawless fish that survived today called the lampreys.0677

And lampreys have vertebrate that are made of cartilage as did other early vertebrates.0687

Lampreys look somewhat like eels, and they are parasites that feed off other fish. They suck the blood from other fish.0692

The early vertebrates were somewhat along this line. They were jawless, and they probably had vertebrates that were composed of cartilage.0701

The vertebrate animals that developed later in evolution had jaws.0714

And they also had mineralized skeletons, which is what most vertebrates in existence today have.0718

And by mineralized skeleton, I mean bone that contains calcium, so it is calcified- a very strong structure.0725

I mentioned that the lampreys are jawless.0738

Vertebrates with jaws are known as gnathostomes, so these are vertebrates that have jaws.0741

And gnathostomes are believed to have arisen about 50 million years after chordates originally arose during the Cambrian period.0754

And then, evolution continued on for hundreds of millions of years and resulted in the diverse array of vertebrates that we have today.0765

And we are going to be talking about six major groups of vertebrates.0773

Now, I want to explain the reptiles in more detail because I mentioned six major classes, and then, you see, OK, there is five.0778

There is Amphibians, Chondrichthyes, Osteichthyes, Reptiles and Mammals.0787

Many groups also or many texts include Aves as a separate class.0794

In a lot of system, you are going to see Aves. However, Reptiles are what is called paraphyletic class.0802

That means that they do not contain all of the descendants of a common ancestor.0810

So, to somewhat rectify the situation, some classification systems include Aves as part of Reptiles.0818

So, you can count it as six with Aves, or some text you will see just use five and place Birds as part of Reptiles.0825

Again, when we talk about the Protists, and when we talk about the Fungi,0837

classification systems are undergoing constant change based on molecular evidence.0844

An earlier classification was based on life cycles and morphology, physiology and biochemistry.0848

And those do not always align with what we are finding out according to molecular biology regarding the evolutionary relationships of animals.0855

The important thing is just to note the characteristics of each group.0864

And we are going to start with the chondrichthyans, and members of this class that you are probably familiar with include the sharks and rays.0868

Chondrichthyans have skeletons that are primarily made of cartilage. Also, this group is ectothermic or cold-blooded.0878

You have probably heard the term cold-blooded, and this means that the animal regulates its body temperature externally.0889

The body temperature of an ectotherm is based on the environment rather than regulating their body temperature internally like a warm-blooded animal.0904

Warm-blooded animal, we call that animal an ectotherm, or we say it is endothermic. It is not endotherm, it is endothermic.0914

These are warm blooded animals.0923

As a representative organism from this class, we are going to focus on sharks.0929

Sharks are predators, and they have a lateral line system. A lateral line system is something you will also hear about with fish.0933

Most fish have a lateral line system. Some amphibians have it, as well.0944

And what a lateral line system is, is a sensory system that allows animals to detect movement, vibrations and changes in pressure in the water,0948

so allows for detection of movement in the water.0957

For a predator, this is very useful. It is useful for a prey, too, because you know if something is coming.0960

In the case of a shark, if they sense something is swimming nearby, that is possible prey.0966

The structure of a lateral line system is that it consists of neuromasts.0971

And neuromasts are receptors that are located in the strip along the shark's head and the sides of its body.0977

Usually, neuromasts are right on the surface or just under the surface of the scales0986

and located near pores to allow for detection of movement and vibrations in the water.0996

Sometimes, people say that this is analogous to the sense of hearing that land animals have.1005

Sharks are not inherently buoyant. Therefore, they must continue to swim.1013

If they do not, they sink.1018

As far as systems in a shark, the respiratory system, well, gas exchange is via gills.1021

Oxygen is pulled in from the water via gills, and then, carbon dioxide is removed from the animal via the gills.1031

Some groups of Chondrichthyans lay eggs and are, therefore, what is called oviparous. These are animals that lay eggs.1045

We will talk later about groups of animals that are viviparous meaning that the offspring develop within the female, and the young are born live.1058

They are not contained within an egg, and that is viviparous.1071

The young are born already developed. They are not contained within an egg.1078

The next group that we are going to cover within of the vertebrate chordates are the bony fishes or Osteichthyes.1090

Osteo means bone, so that is where the name comes from.1101

Now, some scientists consider Osteichthyes to be a superclass not a class.1105

And then, they further divide the bony fish into two classes, which are the ray-finned fishes and the lobe-finned fishes.1110

Or other classification systems just group them together as Osteichthyes.1118

Osteichthyes are the largest group of vertebrates.1125

And both the ray-finned fishes and the lobe-finned fishes have skeletons that are made of mineralized bone, not cartilage.1128

So, skeleton is composed of mineralized bone, meaning that the bone is calcified. It contains calcium.1137

Gas exchange occurs via gills in fish, and the gills are covered by a structure called an operculum.1147

The operculum is a bony flap that covers the gills, and the function is to help keep the water moving through the gills and to protect the gills.1158

Fish draw water into their mouth, and it exits across the gills; and gas exchange can occur.1170

Oxygen can be grabbed from the water and CO₂ can be released out.1175

We talked about how sharks have to keep swimming, or they sink.1180

By contrast, fish can stop swimming, and they will stay afloat because they are buoyant.1184

However, fish have...so, this is one structure they have, and another structure that they have is a swim bladder.1190

Now, remember they are already naturally buoyant. However, the swim bladder allows them to control their buoyancy.1199

So, it is an air sac. It is an air sac that allows fish to control their buoyancy.1210

When the air sac is filled with gas, the fish will rise. When gas leaves the air sac, the fish will sink.1220

Like sharks, bony fishes have a lateral line system.1229

Now, let’s get into these two separate groups of fish.1234

The ray-finned fishes are what you think of probably when you think of a fish.1238

These are fish like salmon or goldfish, minnows, halibut, so the vast majority of fish are ray-finned fish; and this is in contrast to the lobe-finned fish.1247

The lobe-finned fish have fins that look different than what you would typically think of.1262

And the difference is that they have fleshy muscular fins that are supported at the base by bones and muscles, so fleshy muscular fins.1270

These are thought to have been in an evolutionary sense, precursors to limbs of terrestrial animals.1286

This extra support at the base along with their shape, their shape is more cylindrical instead of the flatter, thinner shape with the fins on a ray-finned fish.1294

So, it is more of a cylindrical shape, and what the theory is, is that the lobe-finned fish would sometimes actually crawl out of water to find food.1304

And eventually, through evolution, these fins developed into limbs that allowed for a fish to walk in on land.1316

There are only a few groups of lobe-finned fish that still exist. One of these are the lungfishes.1326

Lungfishes have both gills and lungs, so they could obtain oxygen from the water or from the air.1337

And again, this is the type of fish that may have been the predecessor to land animals.1344

And we are going to talk now about some different groups of land animals.1352

Tetrapods are vertebrates with four limbs or who descended from an ancestor with four limbs.1359

Now, snakes are tetrapods. They do not have these limbs anymore, but they were lost secondarily during evolution.1369

They are believed to be descended from this ancestral four-limbed organism.1377

And when I was talking about the lobe-finned fish, it was probably an ancestral lobe-finned fish that left the water occasionally to search for food,1381

and over many generations, gained an advantage by having those fins developed into limbs that1390

allowed the animal to move around on land and seek food there and survive in that environment.1396

We are going to be focusing on the Amphibians, the Reptiles, and the Mammals,1403

the three groups within the superclass tetrapod and then, focusing on these classes.1407

Again, Reptiles could be split into two.1415

You could have the Reptiles and the Birds, class Reptilia and class Aves, or some books will just group these all as Reptiles.1419

We are going to start out by covering the Amphibians.1429

The Amphibians include the frogs, salamanders and newts and another group called the caecilians.1434

Amphibian: let's breakdown the word, so amphi means double, and bio means life.1443

What this word is referring to is double life, and it gets the name from the fact that some members of this group undergo a dramatic metamorphosis.1450

And they live one part of their life cycle in the water where they will have a certain morphology, and then, they will change.1461

They will go through a metamorphosis and live another part of their life cycle on lands, so it is like they have two different lives.1469

Amphibians are more closely tied to the water than the other tetrapods that we are going to discuss, and this is for several reasons.1475

Most species of amphibians have moist skin, so it is soft, moist skin; and it is very smooth, which allows it to be permeable to gas.1484

This is important because in amphibians, skin often does play a role in gas exchange.1497

So, although, in amphibians many have lungs, the lungs are not as efficient as they need to be for gas exchange.1502

They cannot take on the whole load of the gas exchange, so skin also plays a role in gas exchange.1511

Amphibians live primarily in fresh water, although, some species can survive in brackish water.1517

Amphibians are ectothermic, so remember, that means they are cold-blooded.1523

As adults, amphibians have a three-chambered heart, and another reason that they are tied to the water is that their eggs dry out easily.1529

They have only a thin covering, so they can become desiccated pretty quickly.1539

The eggs must be laid in water or in a moist environment so that they do not dry out.1546

We are going to start out with order Urodela. This is one order of amphibians, and these are the salamanders and newts.1551

These animals have four limbs, and the front and hind limbs are roughly similar in size. They also have a tail in adulthood.1569

Some species in this order are solely aquatic, but many live on land.1577

Those that do live on land, live in areas where there is moist soil, or they will stay under a log or leaves because again,1585

they need to stay moist for their skin to function correctly in gas exchange.1593

These are predators. They feed on animals like insects and worms.1598

They undergo metamorphosis, so they do go from a larval form to an adult form; but it is not as dramatic as what we are going to talk about with frogs.1603

Some species have lungs- those that live on land. Others have gills, and fertilization occurs internally.1615

The next group we are talking about are the caecilians or order Apoda. These are legless animals.1624

They look similar to worms, although, they are not worms; and they evolved from an ancestor with legs and lost their legs along the way in evolution.1641

And this fits their lifestyle because these animals are burrowers, so they like to burrow down and legs are not needed for that and, in fact, could inhibit it.1652

Most of these live in tropical areas. They are found in tropical areas where they burrow down into moist soil in South America, Africa and Southeast Asia.1664

The next group that we are going to talk about includes the frogs, so this is order Anura.1676

Frogs like salamanders have four limbs, but the difference is their hind legs are stronger. They are more muscular.1691

They are very well-developed, and that allows them to hop along very well.1697

Fertilization is external and requires a wet environment or moisture to prevent the eggs from desiccating.1702

On the larva in frogs that you are probably familiar with are called tadpoles, and tadpoles have gills and a tail.1709

They live in water, and they are herbivores. This is in contrast to an adult frog.1720

The frog undergoes a metamorphosis and changes into its adult form.1731

And during this time, the gill and tail is reabsorbed into the frogs body, and legs and lungs form, so replacement of those systems.1738

In addition, the GI tract undergoes changes because although, the larva are herbivores, the adults are predators, and they are carnivores.1747

So, adults develop legs, lungs. They live on land, and they are predators.1757

We talked about the moist skin where gas exchange occurs. A little more detail about that, frogs secrete mucus.1770

They have mucus secreting cells in their skin glands, so they secrete a mucus layer.1778

And some frogs also secrete toxins that are on their skin, and that is a protection from predators.1784

Frogs vocalize well particularly the males as vocalization is important for mating.1792

So, those are the groups of amphibians, and one of the major points is that they undergo a metamorphosis.1800

The other major point is that they are intimately tied with the water because1806

they need it to keep their skin moist for gas exchange and for reproduction.1811

Now, let's contrast that with the reptiles, the next group we are going to cover, and I am going to cover birds separately.1817

We are just focusing on other reptiles like lizards and snakes and turtles right now.1824

Reptiles are well-adapted to life on land. They first appeared about 350 million years ago.1830

They are the first part of a group that we are going to cover called the Amniotes.1838

Reptiles are amniotes. Birds are amniotes, so birds, reptiles and mammals.1844

Dinosaurs were also amniotes.1857

The amnion is a membranous sac that protects the embryo, so it contains fluid.1863

You might have heard of amniotic fluid. It protects the embryo.1868

Also, amniotes have other components. They provide nutrition to the developing embryo.1872

They allow for the elimination of waste.1881

We are going to talk more about these other groups, and remember, reptiles are the first amniotes.1885

Now, reptiles are more well-adapted to land than amphibians. They can live their entire life cycle on land, and why is that?1892

Well, one reason is they have scales that are thick and made of keratin, so they have scales that are keratinized.1900

And remember, keratin is the same material that is found in our nails, and this is in contrast to the moist, more delicate skin of an amphibian.1910

The scales, however, cannot grow.1921

So, as a reptile needs to grow, they are going to shed their scales or molt and then, replace this covering as they grow.1922

Another difference between reptiles and amphibians is that the eggs of reptiles are very well-protected.1933

With mammals, the eggs are protected because the fertilized egg just remains inside the mother's body and develops there.1941

So, the mother's body provides protection.1950

Reptiles lay these fertilized eggs. The fetus develops inside the egg, but the egg has a very thick covering or casing.1952

The eggs are protected by shells. They do not have to be in the water.1962

Covering some specific groups of reptiles, reptiles include lizards,1967

a group that you may not be familiar with called the tuataras, turtles, snakes, crocodiles, alligators and so on.1975

Fertilization is internal, and I have mentioned this before.1991

And this means that the sperm is deposited within the reproductive tract of the female, and this makes sense.1995

With the amphibians, some amphibians like frogs have external fertilization, so the sperm is deposited on the eggs externally.2000

But, since reptilian eggs have this thick casing, the sperm would not be able to penetrate that thick casing.2007

So, this fertilization needs to occur before that thick casing develops, this hard shell.2014

Reptiles are cold-blooded, so they are ectotherms.2019

If you see a lizard, you might see it laying out in the sun to warm up, or if it is very hot out, they will go and seek a cool environment.2027

That is their way of regulating their temperature. They have to do it externally.2033

Gas exchange in reptiles is via lungs, and reptiles have a three-chambered heart.2038

The three-chambered heart consists of two atria and one ventricle, and the ventricle is partly divided.2048

So, let's talk about some of the different orders of reptiles.2058

The first order we are going to talk about are the Crocodiles- Crocodilia, not just the crocodiles. Crocodiles and alligators are members of this group.2062

We have four major orders. The first one are the crocodiles and alligators.2075

The next are Sphenodontia, and these are the tuataras that you may not have heard of.2080

There is only a couple species left, and they both live in New Zealand. They look a lot like lizards, so New Zealand and similar to lizards.2090

The next group, which has about 8000 species in it, are order Squamata. These are lizards and snakes.2102

Lizards have four limbs, so it is obvious they are tetrapods.2111

I mentioned before, snakes are also tetrapods, but their legs were lost during evolution; so snakes are good burrowers.2115

Interestingly, snakes retain remnants of these ancestral limbs in the form of a vestigial pelvis.2123

Snakes are carnivores, so they are capable of swallowing large prey because they have a very flexible jaw that can open widely.2132

The Testudines include turtles and tortoises. There are about 300 species in this group.2142

And what is distinctive about them is the hard shell that they possess that provides protection.2149

If under threat, a turtle will withdraw its head and limbs so that it is well-protected within the shell.2155

Some members of this order live on land. Others live in aquatic environments.2161

Now, we are going to go on and cover the birds, which are sometimes group as reptiles.2169

So, I am going to go ahead and cover these separately because they have many distinctive features.2174

Of the living animals, birds are the ones that are thought to be most closely related to dinosaurs.2179

I am going to take a minute now and talk about dinosaurs.2186

Although, we are really mostly focusing on species of animals that are still in existence, we are going to talk a little about dinosaurs.2189

Birds are warm-blooded, so they are endotherms; and it used to be thought that dinosaurs are cold-blooded. They are like reptiles.2196

However, new discoveries in the past decade have led scientists to believe that some species of dinosaurs were warm-blooded, as well.2206

Dinosaurs, some were herbivores, some were carnivores, and they radiated out to occupy many niches on land.2216

They varied from quite small to huge and were found on every continent during their peak.2222

There are multiple theories about why the dinosaurs became extinct.2229

One that has gained more support in recent years is the theory that there was a large2233

asteroid that collided with the earth about 65 million years ago during the Cretaceous period.2237

And the result of such a large asteroid hitting the earth is that all kinds of dust and debris were thrown up into the air,2250

which then, blocked sunlight from reaching the earth.2257

The results of that is that photosynthesis could not occur. Plants could not grow.2258

And since plants are at the bottom, they are at the base of the food chain and other photosynthetic organisms,2264

when photosynthesis is limited, it is going to have a severe effect on all organisms.2270

So, that is one theory on why the dinosaurs died out.2278

Birds are believed to have evolved from a group of dinosaurs called the theropods, and theropods were bipedal.2281

They moved on two feet, and they had three toes like birds. Most of these were carnivores.2290

So, it is believed that birds evolved from this group of dinosaurs.2302

They evolved to adapt to many diverse environments. You will find birds in very extreme cold like the Arctic or in hot dessert environments.2308

They have eggs that are covered with shells and are well-protected.2318

While some birds like ostriches cannot fly, most do, and when you look at a bird,2323

it is not just the feathers and wings but many, many other features that the bird has that allow it to fly, that are adapted to flight.2329

Almost any system or structure on the bird's body, if you look at and think about it, in some way has been adapted to allow for flight.2336

For one thing, birds have a high metabolism. In order to fly, it takes a lot of energy.2345

It takes a lot of oxygen. It takes a lot of glucose, so birds have a respiratory system that is very efficient.2357

They have an efficient respiratory system that does an excellent job of extracting oxygen from the air.2365

Their lungs are structurally different than those of mammals, and birds lack a diaphragm.2377

A diaphragm is a sheet of muscle that when you breathe, it helps to move air in and out of the lungs.2382

Birds lack a diaphragm. Birds have a four-chambered heart.2389

Again, flight requires a lot of energy. It requires oxygen, so you are going to need a very efficient respiratory and circulatory system.2393

So, this four-chambered heart has two atria and two ventricles and is extremely efficient in supplying oxygenated blood to the body.2402

Birds, as I mentioned, are endothermic. They are warm-blooded, and so, they can regulate their body temperature internally.2417

And in fact, due to their high metabolism, they have a body temperature that is slightly higher than the body temperature of mammals.2425

The bones of birds are strong but lightweight, and to make them lightweight, some areas of the bird's bones are even hollowed out.2432

So, there are hollow areas in the bones.2446

Certain bones such as the lower vertebrate and those with the pelvis are fused together, and the sternum serves as a keel.2448

The pectoralis muscles, those are the chest muscles.2458

The chest muscles are anchored to the sternum to the breast bone.2462

And those chest muscles are very well-developed because they power the wings they allow for flight.2467

Feathers: feathers are made of keratin. We already talked about keratin when we talked about the scales on the other reptiles.2477

Feathers allow for flight, and they also provide insulation to the bird.2486

In addition to the light bones that I mentioned, birds are designed to not have anything2491

that is not absolutely essential because that extra weight is a big problem in flight.2498

For example, birds have a beak, but they lack teeth, so that helps them to be lighter.2503

They also do not have a urinary bladder, and in many species, the females only have one ovary instead of two.2507

Again, it is all about not carrying unnecessary weight when they are flying.2515

Birds have relatively large brains for their body size. They are good communicators.2520

Considering their flying, they also need to have good sight and navigational skills.2527

They have good communication skills, and birds are known for their different songs that are used for communication or mating.2535

Again, many of the main features of birds have to do with allowing them to fly, and that goes beyond just having wings and feathers.2544

Now, we are going to move on to cover the mammals.2553

Mammals are endotherms, so they are warm-blooded. They also have hair, and they produce milk to nourish their young.2556

Milk is produced in the mammary glands, which are modified sweat glands, and milk is a liquid that is very rich in proteins and fat.2564

Mammals have high metabolism, and we are going to go into detail about respiratory and circulatory2575

and all these different structures when we talk about physiology but just briefly, the respiratory system.2582

Air is moved into and out of the lungs by the diaphragm.2589

So, the diaphragm, I said birds do not have a diaphragm. It is a sheet of muscle that aids in respiration.2593

Mammals have a four-chambered heart, again, two atria and two ventricles.2613

Another unique thing about mammals is that their teeth are highly specialized, and they are what is called diphyodont.2625

A diphyodont has two sets of teeth. The first set is, in some cultures, called baby teeth or milk teeth.2636

And then, those teeth are lost and then, replaced by a second set, which are the adult set.2646

And as I mentioned, the teeth are specialized for different functions.2652

For example, the incisors and the canines that are located in the front of the mouth are used for cutting and tearing.2655

Then, if you go a little further back, you will see the premolars. They are used to shred up the food.2663

And then, finally in the back, the molars, which are used for grinding.2668

Mammals have large well-developed brains, and fertilization in mammals is internal.2673

Fossils for mammals date back to the Triassic period about 200 million years ago.2682

And mammals are believed to have descended from a group called the synapsids.2688

These were amniotes, and they had a characteristic opening called a fenestra.2694

They had a fenestra, and a fenestra is a hole or an opening behind the eye socket.2701

Fenestra is just a general name for opening, but the opening behind...2706

They had a fenestra that was actually located behind the eye socket.2710

And it is thought to be a place where the jaw muscles anchored, but it is just something that distinguishes this group, so fenestra behind the eye sockets.2714

Mammals are thought to have evolved from this group around the Jurassic period, and for a time, mammals did coexist with dinosaurs.2730

But they were, sort of, limited, and it is thought to be because dinosaurs were filling a lot of the niches in the environment.2738

Once the dinosaurs died out, mammals went through an enormous adaptive radiation.2745

And they filled a lot of the environmental niches that had been occupied by dinosaurs, and now, they are found in every continent on land, in the sea.2750

The only flying mammal is the bat, but there are other mammals such as whales and dolphins that have adapted to life in the sea.2760

In this case, although they are tetrapods, evolved from a four-limb animal, what happened is that the four limbs became adapted to swimming.2767

And there are about 5000 species of mammals.2780

And they are divided into the three taxonomic groups that we are going to discuss: the monotremes, the marsupials and the eutherians.2785

We are going to start with the monotremes. There are only five species of monotremes left in existence, and most of them live in Australia.2794

These include the platypus and echidnas, which are the spiny anteaters, and what is very unique about monotremes is that they actually lay eggs.2807

When we think of mammals, we do not think of them as laying eggs, but this group does.2816

So, they lay eggs. They do not deliver their young fully live outside of an egg.2820

It is unique to this group among the mammals.2828

A group of mammals that you are probably more familiar with are the marsupials. Marsupials include kangaroos, wombats, koala bears and many others.2832

And the distinctive characteristic of this group is that they have a pouch where the fetus finishes its development.2843

Marsupials have a relatively short gestation period, so they deliver their young early in their development.2850

At the point that a marsupial offspring is born, the hind legs are usually just small buds.2857

What happens is the developing new born is born. It is tiny.2862

It has these little hind legs or buds really, and then, it uses the four limbs to climb up and reach the pouch.2868

Once inside the pouch, the marsupial will attach to a nipple and feed of milk.2877

It will nurse there and stay in the pouch and finish out its development there.2884

Now, marsupials evolved in isolation in Australia, and when we talked about evolution, I mentioned that.2890

And because of this isolation, marsupials went and radiated out, filled all kinds of niches in Australia,2899

while in the rest of the word, placental mammals which are called eutherians went and developed similar niches.2908

As a result, there are many examples of convergent evolution among marsupials and placental mammals, so convergent evolution.2913

I mentioned some of those in the evolution lecture, but just to recap, an example would be the wolf in North America, the placental wolf,2923

And then, if you look in Australia, you would see an animal that looks pretty similar or has many similar features called the Tasmanian wolf.2933

The flying squirrel in North America and the sugar glider in Australia,2950

although, they have many superficial similarities, if you looked at one of these marsupials,2960

they are actually more closely related to other marsupials than they are to their counterpart placental mammal.2966

We are going to, now, go on and talk about some of these placental mammals and why they are in a different group.2975

To start with, eutherians is one name, but you will often hear them called placentals or placental mammals.2983

Although, marsupials have a placenta, it is not as well-developed as it is in this group of mammals, and that is how they got their name.2989

The placenta in placental mammals is a very well-developed structure.2997

And what the placenta does in utero is it provides the fetus with a means for gas exchange, nourishment and a means to eliminate waste.3003

Eutherians have a longer gestation period because they complete their gestation in utero,3023

in contrast to marsupials that are born much earlier and complete their development in the pouch, so a much longer gestation period.3030

Some orders of eutherians include Carnivora. Examples here would be dogs and cats, Rodentia, so rodents such as rats, mice and squirrels,3049

also, the Cetaceans, and these include aquatic dwelling mammals such as whales and dolphins,3075

and finally, the group we are going to focus on, which are the Primates.3090

Humans belong to this order as do many other animals, so we are going to talk about the Primates in more depth right now.3094

Primates include monkeys, orangutans, lemurs, chimps, gorillas and humans among others.3103

Rather than claws, primates have flat nails, so we are talking about some differences between Primates and other groups of mammals.3120

They have no claws, so they lack claws. Instead, they have nails, flat nails.3129

They also have hands that have an opposable thumb, and most primates are good climbers.3136

And some species spend the majority of their time up in the trees in arboreal habitat.3144

It makes sense that an opposable thumb would allow for grasping. The hands of primates have good dexterity and are highly innervated.3150

The fingers are very sensitive and highly innervated, and this allows primates to be very successful at performing tasks requiring fine motor skills.3160

Primates have a flatter face than most other animals, and their eyes are forward looking, so closed-set forward-looking eyes.3172

And this is an advantage up in the trees, climbing around in an environment where you need to have good depth perception.3188

Those closed-set eyes allow for good depth perception and in general, good visual acuity and hand eye coordination. They are very dextrous.3194

In addition, most primates are quite social, and they care for their young for extended periods- extended care for their young.3208

Primates have large brains particularly the cerebrum. The forebrain is well-developed, and primates tend to be highly intelligent.3226

They are capable of learning. They have very flexible behavior relative to other groups of animals.3240

Humans are primates with especially large brains and that walk upright.3246

Although, other primate groups can use tools, humans do so in a way that is much more complex than other members of the primate group.3252

So, today, we looked at a lot of different chordates starting with the invertebrates and then, finishing out with the vertebrates including the primates.3262

Now, we are going to go ahead and do some questions to review the lesson.3271

Example one: list the four distinguishing characteristics of chordates.3275

At some point in development, all chordates have four things, four structures.3281

One is a notochord. Remember that this is a rod-type structure that is located dorsal to the GI tube and ventral to the nerve chord.3287

It is not a vertebral column, but it can function similarly in invertebrate chordates.3298

The second structure that chordates all have at some point is a dorsal hollow nerve chord.3305

This nerve chord differentiates into the CNS, which includes the brain and spinal cord.3315

Next, all chordates have pharyngeal clefts and arches, and these differentiate into various structures depending on the animal.3322

It could be a structure to allow for filter feeding. It could be gills.3335

It could be parts of the ear, so it depends on the animal.3339

And finally, all chordates have a post-anal tail at some point in development.3346

So, these are the four structures that are characteristic of chordates.3352

Match the following descriptions to the correct term.3357

The only group of mammals that lays eggs- so, recall there is three groups of mammals that we covered.3361

Marsupials do not lay eggs, although, they are mammals. Reptiles- no, amphibians.3367

It is the monotremes, so the monotremes are mammals, but they lay eggs.3374

Two: live a part of their life cycle in water and a part on land. They have moist skin that is used for gas exchange.3381

So, recall that amphibian means two lives or double life, and that is because amphibians like frogs live part of their life cycle in the water and part on land.3391

And they do have moist skin that is used for gas exchange in addition to the lungs. Therefore, no. 2 is C.3401

Three: ectotherms with scales made of keratin. Their eggs are covered by thick protective casings.3411

Marsupials are not covered with scales. It is the reptiles.3419

And remember that those thick scales and the covering and casings that their eggs have3423

allow reptiles to live fully on land and not be as dependent on the water as amphibians are.3431

Finally, mammals who complete their development in the pouch- those are the marsupials such as kangaroos.3440

The young are born early in development, make their way into a pouch where they nurse.3445

They feed on milk and continue their development protected within a pouch.3453

What are three ways in which a bird's anatomy is adapted to allow for flight? Well, I am going to expand that out to anatomy and physiology.3459

So, we are going to talk about structure and function here, and that would include the existence of feathers.3468

These are made of keratin, and they also provide insulation. They have wings, so those are the obvious two.3475

Others are the fact that they have light bones. Their bones are strong but light.3483

Certain bones such as the pelvic bones are fused, and lower vertebras are fused.3488

They also have no teeth. This allows them to be lighter to allow for flight.3497

There are other ways in which birds are lighter such as many species only have one ovary, which allows them to be light.3505

Physiologically, birds have a fast metabolism. They need a lot of energy, a lot of oxygen also, to be able to fly, oxygen and glucose and forms of energy.3512

Therefore, they have very efficient respiratory systems and circulatory systems.3528

So, I listed many more than three, but you only had to remember three out of these ways in which a bird is adapted to allow for flight.3544

Finally, example four: a vertebrate animal living in a marine environment has the following characteristics.3556

Ectothermic, so that means it is cold-blooded, a skeleton composed of cartilage.3563

It is living in the water- cold-blooded. It has a cartilaginous skeleton and a lateral line system.3568

Well, this could describe the bony fish except for the fact that we are talking about a skeleton that is composed of cartilage.3575

This sounds like a chondrichthyan like a shark, so this is a chondrichthyan or a member of the phylum Chondrichthyes.3581

That concludes this session on the vertebrates at Educator.com.3604