AP Biology Invertebrates

Welcome to Educator.com.0000

We are going to begin our discussion of animals with the invertebrates.0002

During this section and the next one, we are going to be focusing on the major0005

phyla of animals that you should be familiar with for the Advanced Placement Exam.0009

But before you go on, you should watch the lesson on classification if you have not done so already because this lesson0014

assumes that you are familiar with the discussion of symmetry, germ layers, protostomes, deuterostomes, and other topics.0022

So, make sure that you watch that if you have not done so already, and now, we will get started talking about invertebrates.0030

Animals are eukaryotes and, therefore, are part of the domain Eukarya.0038

They are multicellular heterotrophs, and most animals on earth are invertebrates.0044

Invertebrates lack both a backbone and a spinal cord.0052

Now, I am going to cover multiple phyla today, and these contain all invertebrates.0057

There is one phylum, Chordata, that does contain a few groups of invertebrates, but most chordates are vertebrates.0063

Therefore, I am going to cover Chordata under the vertebrate section including those few groups of chordates that are invertebrates.0075

We are going to start out with some of the most primitive animals, which are members of the phylum Porifera.0085

These are the sponges, and some systems divide the sponges into two different phyla.0092

I am just going to keep them together as a single phylum today.0098

They are very primitive animals, and they are found in water. They live in aqueous environments.0103

Sponges are non-motile. Another word for non-motile that you should be familiar with is sessile, so this means non-motile.0109

Sponges lack body symmetry. As we discussed earlier on, some animals have radial symmetry.0118

Some have bilateral symmetry. Sponges do not have either.0125

They are not symmetrical. They have two layers of cells.0130

Therefore, they are diploblastic, so the two layers are an ectoderm and an endoderm.0136

Between the two layers is a layer called the mesohyl. This is a gel-type substance that lies between these two layers of cells.0149

These are acoelomates. Recall that this means that they are animals that lack a body cavity, so they are acoelomates.0164

They also do not have specialized tissues or organs.0175

Sponges have certain cells that are specialized to various functions, so they have specialized cell types.0179

And we are going to talk about some of those, but they do not actually have tissues and organs.0185

Sponges are filter feeders. You may also hear the term suspension feeders.0191

That is the same thing- filter feeders or suspension feeders.0196

And the name tells you what it is that they feed by filtering water and straining it through a structure and then, capturing the bits of food.0200

In a sponge what happens is - here is the sketch of a sponge - water will enter the sponge through pores.0217

And then, it will exit through an opening called, this is the osculum.0227

Water is going to enter, make its way through. Nutrients will be trapped within the sponge and then, exit out the osculum.0234

There is a central cavity within the sponge.0247

If you thought of this as a cross-section, and I just sliced it down as a cross-section,0249

and I could see inside, what I would see is a central cavity called the spongocoel.0254

Water is going to enter that cavity and then, pass through the spongocoel and exit via the osculum.0262

What this shows here is just a blown up view of cell types and structures that are located along the mesohyl.0272

One type of cells is actually not shown here but that you should be familiar with are called amoebocytes, and they are motile using pseudopods.0289

And you will recall earlier when we talked about protists, we talked about amoebas, and amoebas had pseudopods; and so, the name here is similar.0304

Amoebocytes are motile cells. They use pseudopods for motility, so they are motile; and they have multiple functions.0313

They are located in the mesohyl, and they are important in digestion.0325

They will digest the food. They, then, carry these nutrients to other cells in the sponge.0330

They produce fibers that are found between the two cell layers, and in fact, this shows you some of these fibers .0334

Some of these fibers are, sort of, pointy and sharp, and they are called spicules.0342

The spicules are produced by the amoebocytes and help to provide some support for the organism, for the sponge.0346

That is one type of cell within a sponge.0353

Another type are choanocytes or collar cells, and that is what is shown here- choanocytes or collar cells.0357

And as you can see, they have flagella, and their function is to help move water through the sponge.0370

They line the spongocoel and have flagella, so then, when the water enters, these cells help move the water through the sponge to exit out the osculum.0376

So, that is the structure of the sponge. As far as reproduction, sponges can reproduce asexually or sexually.0389

Asexual reproduction occurs through what is called fragmentation, so it is via fragmentation.0396

And this means the entire organism can be regenerated from just a fragment or a piece.0410

A certain fragment or a piece of the sponge can regrow into an entire sponge.0418

As far as sexual reproduction, sponges are hermaphrodites. That means that a sponge could produce both sperm and egg.0425

They can function as either a male or a female.0438

So, at one point in the life cycle, a sponge may produce sperm. At another, it may produce eggs.0441

Sperm are deposited into the water and then, carried by the water over to another sponge where it could, then, fertilize the eggs.0447

This type of fertilization is called external fertilization.0455

Internal fertilization means that the sperm are directly deposited into the reproductive tract of the female.0458

After fertilization takes place in the eggs, which are present in the mesohyl, zygote develops, and these are flagellated.0465

They eventually developed into larvae, and these larvae can attach to a rock or some other substrate where they will develop into an adult.0474

That is the first phylum that we are going to...0483

OK, the phylum Cnidaria includes the jellies, which are commonly known as jellyfish, Hydra, corals and sea anemones.0486

They are found mostly in marine environments, and they have true tissues.0495

We are seeing an advancement from the sponges, which lack tissues and organs to cnidarians that have tissues.0501

There are two possible body plants: polyp and medusa.0511

We will talk more about these in a second, and the nervous system of the cnidarians consists of what is called a nerve net.0516

Now, the polyp body plan is non-motile, whereas, medusa is motile.0525

Both body plans involve a central cavity called the gastrovascular cavity, and both include tentacles.0535

There is only one opening leading to the body cavity, so there is not a separate mouth and anus.0549

Food enters the same cavity as waste products leave through.0556

When there is water in the gastrovascular cavity, it can act as a hydrostatic skeleton that contractile tissues can push against, they can work against.0561

So that is called a hydrostatic skeleton - so hydro meaning water - when it is filled with water. It functions that way.0573

Cnidocytes are cells that contain stingers, which are called nematocysts. Their cells contain nematocysts, and these are stingers.0583

The stingers are used to attack prey, and they sometimes contain toxins.0603

The name Cnidarian comes from the Greek word cnidos, which means stinging nettle, and these are thread-like stingers.0608

You might have heard of people getting stung by a jellyfish.0618

And the stingers can cause anything from just a little bit of pain, a slight rash to - depending on the species - even being deadly.0621

So, there is a big range in how toxic the poisons within the stinger are.0629

There is no central nervous system, as I mentioned. There is a nerve net but no CNS.0636

There are four major classes of cnidarians.0641

The first class is the Anthozoa. These include corals and sea anemones.0646

These are non-motile, and they have, then, the polyp body plan which is non-motile.0661

Corals have calcified external skeletons that are left behind as fossils. These are only marine organisms, so there is no medusa stage.0667

They are non-motile, and they have the polyp body plan, so non-motile, and then, I will just put polyp; so that is one class.0677

The second class are the cubozoans, so class Cubozoa.0685

These include the box jellies, and they have the medusa body plan.0693

And box jellies have stingers with strong toxins in them, and these are marine organisms with complex eyes; so that is the second group.0701

The third group are the Hydrozoans. An example would be Hyrdas.0713

Their life cycle includes both a polyp stage and a medusa stage, so I will just put polyp and medusa stages here because they have both.0721

In the polyp stage, Hydras live in colonies, and they reproduce asexually by budding.0738

Within these colonies, different polyps have different functions. Some are specialized for feeding.0745

Others are defenders, and others are reproductive.0750

So, when the polyps reproduce, they can actually produce a medusa, so reproduce via budding to produce a medusa.0755

The medusa stage of a Hydra is free swimming, and they produce sperm and eggs and can reproduce sexually.0774

So, in the polyp stage, it is asexual reproduction through budding to produce a medusa. The medusas are free swimming and reproduce sexually.0782

Hydrozoans are found in both fresh water and marine habitats, and these are usually bottom dwellers.0792

Finally, we have the scyphozoans, so class Scyphozoa, and these include the true jellies.0805

They live in marine environments and spend most of their lives in the medusa stage.0818

Some species do have a polyp phase in their life cycle. Others do not, so I am just going to put "mostly medusa"; so that is the Cnidarians.0825

The next phylum we are talking about is the Platyhelminthes.0837

These are flatworms, and this is the first group that we are covering that is triploblastic meaning that it has three layers of cells.0842

We have already talked about animals with an ectoderm and an endoderm.0851

Flatworms have a third layer called the mesoderm, so this is a recurring theme of going from simple to more complex.0859

A second way in which flatworms are more complex in the groups that we have talked about previously are that they are bilaterally symmetrical.0868

They have bilateral symmetry, and bilateral symmetry is much more effective for motility than radial symmetry or being asymmetrical- lack of symmetry.0878

So, bilateral symmetry, and another advancement is cephalization meaning that there is an anterior head end where the sensory organs are concentrated.0897

So, flatworms exhibit cephalization. These are acoelomates, so they do not have a body cavity; and as their names suggests, they are very flat.0909

This means that their cells are in contact with the environment.0920

And that allows for flatworms to take in nutrients via diffusion and for gas exchange to occur through diffusion.0924

The GI system, they have only one opening that is used to take in food and to remove waste, so no separate openings for the mouth and anus.0936

The digestive cavity contains a bunch of branches that deliver food to various parts of the flatworm, but there is no true digestive or circulatory system.0946

So, one opening and various branches to deliver nutrients, and nutrients are taken in via diffusion.0959

The excretory system is a network of small tubes, so excretory system.0968

It is a network of small tubes that lead to docks that connect to the outside of the body, so tubes going to docks,0977

going to the outside of the body to get rid of waste and to excrete waste and fluid that needs to be removed from the body.0988

And these are called protonephredia.0999

Flame cells are ciliated cells that move fluid and solutes through these tubules to get rid of the excess fluid, to get it out of the body.1008

The nervous system is a bit more complex than just the nerve net of the cnidarians that we discussed.1021

It consists of longitudinal nerve chords and a pair of ganglia- plus ganglia. Ganglia are clusters of nerves at the anterior end of the organism.1031

Some flatworms also have eyespots that they can use to sense light, and that input can process by the ganglia,1045

so again, cephalization with sensory organs concentrated at the head end of the organism.1051

Flatworms are mostly predators. They eat smaller insects or other small organisms.1057

Some of them feed off other organisms as parasites, and they can range from being very, very small to tens of feet long.1062

And we are going to cover three major classes.1071

The first class that we are going to talk about are the turbellarians, and these are a group of free-living flatworms.1074

They are found in both fresh water in marine environments, and they move using cilia.1089

They reproduce asexually through fission, so they split into two halves and then, regenerate the missing half, so asexual reproduction through fission.1094

They can also reproduce sexually and are hermaphrodites. An example of this group would be the planarians.1112

The second group or class are the trematodes, and these are parasites, so they have a host.1122

And one example are members of the genus Schistosoma.1137

And these cause a disease in humans called schistosomiasis that causes GI tract symptoms like pain, diarrhea as well as fever and chills.1141

Trematodes are flukes, and again, an example would be organisms causing schistosomiasis, and these are members of the genus Schistosoma.1151

Many of the flukes have a primary host as well as an intermediate host.1165

To give you an example of a typical life cycle, what would happen is a larva would enter their human host through the skin.1170

So, let's say that somebody is swimming in a pond that is contaminated with parasitic trematodes, and the larva penetrates the human skin.1180

So, somebody is swimming in this pond. The larva gets in through their skin.1194

Once in the body of the person, of the host, these larvae will make their way up to the blood vessels in the GI tract of the human.1198

In the GI tract, they will mature and reproduce sexually.1206

Then, what happens is, after sexual reproduction, there are fertilized eggs.1212

And when the person whose infected eliminates their waste, their excrement will contain these fertilized eggs.1216

The cycle continues on if that human waste ends up in a body of water because the1224

flukes need their intermediate host, which in some species, the intermediate host is a snail.1231

So if human sewage, human waste ends up in a body of water, and these snails are living around there,1237

then, the fertilized eggs develop into a larva, infect the snails, continue their development within the snail to a more advanced larval stage,1244

and then, the larvae are released from the snail, somebody else goes swimming, the larva can penetrate their skin, and the cycle goes again.1259

So, the primary host is a human host. The intermediate host is the snail.1268

And the snail is required for them to complete the larval stage of development and then, be released before they infect another human host.1272

As you can see, sanitation is extremely important in preventing this disease, and in fact, schistosomiasis is a major cause of dysentery worldwide.1281

OK, that was the second group.1293

The third group are the monogeneans, and the monogeneans are also parasites; but they are mostly parasites of fish, so I will just say fish parasites.1295

Actually, we are going to cover one more group. These two are often grouped together.1315

So, we will just divide it into four, though, and that is Cestoda, the tapeworms.1320

These are also parasites with human host, and like the trematodes, they have an intermediate host.1329

Tapeworms attach to host such as humans via suckers or hooks and absorb nutrients. They do not have a mouth.1336

They absorb the nutrients. They do not take them in through a mouth.1344

And again, like with the trematodes, when the fertilized eggs end up in the human GI tract and are eliminated in the waste,1350

then, they can be passed to an intermediate host where they complete their larval development, and the cycle continues.1362

For example, If a pig eats food that has been contaminated with human waste, it could ingest the eggs from that infected human.1369

Those fertilized eggs - they are actually embryos - will grow into larvae in the pig, and they will form cyst inside the pig muscle.1379

If that pig is used as food, and the pork is not well cooked,1389

a person eating it could ingest these cysts that are, actually, tapeworm larva that have not been killed by heat.1393

So, they will ingest the pork containing a cyst, and within the human, they will grow into an adult tapeworm in the GI tract.1401

And again, then, reproduction could occur. These eggs are passed out of the human, and it continues.1410

Tapeworms can actually grow several feet long inside their human host.1418

The next group are the rotifers or phylum Rotifera, and these are microscopic aquatic animals.1426

They are very small, but they are animals, and they are multicellular.1435

It might just be a couple of millimeters long or even smaller, and they live in both fresh water and marine environments.1440

They are also found in moist soil. They have a complete digestive tract, so that means there are two openings.1446

There is a mouth for the entry of food and an anus for the elimination of waste, so complete GI tract.1454

They have a pseudocoelom, so this means that it is a body cavity; but it is not completely lined by mesoderm the way a true coelom is, and let's see.1464

They are covered by a cuticle. These are also filter feeders or suspension feeders.1484

So, you can see the structure here of this rotifer, and that they use cilia to actually pull water into their mouth.1490

And the name rotifer means wheel-bearer, and they give this name because when the cilia are moving very quickly, it looks like a wheel turning.1500

Again, these are filter feeders. Water is, sort of, brought into their mouth via the cilia.1514

It is ground up by trophi within their jaws, so trophi for grinding food.1521

Some of these organisms reproduce asexually via parthenogenesis, so this is a new term.1534

Parthenogenesis means that the female produces eggs. The eggs are not fertilized.1540

They just grow into female offspring, so unfertilized eggs, and this is asexual reproduction. However, sexual reproduction can occur.1546

The males are very rudimentary. They can even feed themselves.1569

However, they do produce sperm, and those sperm can fertilize eggs; and the fertilized egg forms a zygote that is resistant to desiccation.1573

We talked previously about protists and fungi that switch to sexual reproduction for survival. It is the same situation here.1582

If there are conditions where it is very dry, there is low water, then, the rotifer will switch to sexual reproduction to form a resistant zygote.1590

Once there is more water available, the zygote will develop further.1599

Nematodes are the next phylum that we are going to cover, and these are the roundworms.1608

Nematodes are unsegmented worms, and they are found in both aqueous and moist soil habitats.1613

Some species are free living. Others are parasites, and these are an extremely abundant group of animals.1620

They have a complete GI tract and a pseudocoelom.1627

They have no circulatory system, so lack a circulatory system and have muscles that are all longitudinal.1632

Again, complete GI tract means a separate opening for food and for elimination of waste- two separate openings.1639

Both free living in parasites, but you might have heard more about the roundworm parasites such as pinworms and hookworms.1648

An infection can occur when an individual eats meat that has been undercooked, and that is infected with pinworms or hookworms.1663

An example is Trichinella. This is an example of a roundworm.1672

These cause the disease trichinellosis. These roundworms are parasites that live within the human GI tract and muscles.1678

And they travel through the body through the lymph system causing symptoms such as abdominal pain, diarrhea, fever and muscle pain.1688

You may also hear during your science studies about C. elegans, and C. elegans is a roundworm that has been very well studied.1697

It is frequently used in research, so you should just be familiar with that name.1707

Nematodes reproduce sexually, so these are unsegmented worms.1711

The next phylum contains the segmented worms such as earthworms and leeches as well as another group of worms called the polychaetes.1716

The polychaetes live primarily in marine environments.1727

Members of Annelida can range from just a few millimeters long to the giant earthworms that can be over three feet long.1732

And this is the first group that we are covering that has a true coelom.1740

We have talked about acoelomates and organisms with a pseudocoelom.1745

The annelids have a true coelom, and they also have a closed circulatory system.1750

Let's talk about what a closed circulatory system is versus an open circulatory system.1756

In an open circulatory system, blood is pumped through the body, and it can be in blood vessels.1761

But, at some point, it leaves the blood vessels and enters body cavities that are called sinuses.1768

So, the organs end up bathed in the fluid that contains nutrients or oxygen and takes away waste.1774

But, the blood does not stay contained within a blood vessel the entire way to when it is delivered to an organ.1782

So, in an open circulatory system, blood or hemolymph, which is a slightly different fluid...1792

So, in an open circulatory system, there are sinuses and fluid bathes the organs.1804

Whereas, in a closed circulatory system, the fluid that is carrying oxygen or nutrients such as blood is contained in vessels all the way to the organ.1819

It never leaves the vessels and just goes out into an open space, so annelids have a closed circulatory system.1844

I mentioned that these are segmented worms, and so, there is a structure that is repeating.1853

And there are internal septa that divide these segments up, although, there is communication in delivery of things between the different segments.1860

Muscles are both longitudinal and circular, and annelids are covered by a cuticle for protection outside the epidermis.1871

The nervous system of annelids includes a pair of ganglia, so cerebral ganglia at the head end, and these communicate with ventral nerve chords.1880

The excretory system consists of metanephridia, and these are tubes through which waste can be removed from the blood stream.1903

So, excretory system consists of metanephridia, which are tubes that waste from the blood stream enter and then, are removed from the body.1911

There are three major classes that I am going to cover of annelids. The first are the oligochaetes.1927

Oligochaete means few bristles, and these are covered by sparse bristles and include earthworms, so few bristles. An example would be earthworms.1934

Earthworms live in moist soil, as I am sure you are probably aware, and here is a picture of an earthworm.1955

And they have a very important function as far as human surviving because as they go through the soil, they eat their way through the soil.1964

They take nutrients from the soil and eliminate the waste out the other end through the anus.1973

The result of this is that it actually improves the quality of the soil so that is more productive agriculturally.1980

That is a very important function of earthworms is it increases food production.1988

Earthworms reproduce sexually, and they are hermaphrodites.1994

So, that is the first class, the oligochaetes including earthworms. The second class are the leeches.1999

These are usually found in fresh water environments. They can be up to a foot long.2006

Some leeches are parasites, and they suck the blood from their host.2011

And their saliva contains an anticoagulant so that while they are sucking blood from their host, the blood will not just clot off.2015

It will stay finned out and it will keep on flowing.2023

Some of the leeches use a structure called a proboscis to puncture the host skin, so it punctures the host skin, allows them to feed off the host.2027

And these were used in medieval times by physicians to remove blood from the patients.2044

And in fact, in modern times, we have started using them again in modern medicine. One use is after a limb reattachment.2050

If a limb such as an arm or something, a hand, needs to be reattached to a patient, at first, their veins have not recovered and are not functioning.2058

And without functioning veins, fluid is going to build up. There is going to be an excess of blood and engorgement of that area.2069

So, leeches can be used to remove this excess fluid.2078

Finally, there are the polychaetes that I mentioned before, and these are segmented worms that live in marine environments.2084

So, they live in marine environments.2094

Polychaetes have structures called parapodia that they use for movement, and they are similar to fins, so I will just put "approximately like fins".2098

They are not fins, but they are similar.2111

Some of these are free living. Others are sedentary, and some of these are what is called tube dwellers.2113

They actually construct a tube and then, live inside that.2120

These are the major classes of annelids, and the next group we are going to talk about is a very diverse phylum, and it is the molluscs.2121

Molluscs include octopuses, chitons, snails, slugs, clams, squids.2132

We are going to start out by talking about some of the features that they have in common.2140

Many of these organisms have a soft body with a hard shell. There are exceptions.2145

Some have an external shell. Others such as slugs have no shell.2150

The shells though, for example snails have shells, and it is a calcified shell meaning it contains calcium.2154

It is mineralized, so it is a very hard protective shell.2163

Molluscs are bilaterally symmetrical and have three germ layers.2166

Now, this shows just a generalized mollusc.2171

It, sort of, looks like a snail, but it is just supposed to be a general mollusc so that I could point out the three major parts of the body.2174

The first major part is what is called the head foot region, and if this was a snail, this is the part that you would see sticking out of the shell.2182

It is used for locomotion, and the head region contains sensory organs; so locomotion and sensory organs are contained here.2192

The second region here in light tan is the visceral mass, and a visceral mass contains the internal organs.2209

Here you can see the GI tract.2226

For example, reproductive organs will be located in the visceral mass, and then, above the visceral mass lies the third region, which is the mantle.2228

In many species, this mantle secretes the shell. The space between the mantle and the visceral mass is called the mantle cavity.2243

And in aquatic species, there could be gills. In other species, the space can function more as a lung, for terrestrial species.2258

Many species of molluscs have what is called a radula, and it is often described as a saw-like structure.2270

And it contains teeth that curve backward into the organism's mouth.2281

And these can be used to scrape food such as algae off a hard surface.2285

Or the radula can be specialized depending on the food source that a particular mollusc eats.2289

Most molluscs have an open circulatory system, but cephalopods such as squids and octopuses have closed circulatory systems.2296

Remember in a closed circulatory system, the fluid is enclosed in a tube, in a blood vessel, all the way up until it is delivered to the organs.2305

Reproduction is sexual.2317

Some molluscs are hermaphrodites and have both male and female reproductive structures. Others are not.2319

The excretory system consists of paired nephridia, and the nervous system consists of paired ganglia with nerve chords.2325

Some species have chemosensory organisms including image-forming eyes like with the cephalopods.2345

So, let's talk in more detail about some specific major classes of molluscs.2352

The first class we are going to talk, about an example is right here, Gastropoda. These are the snails, limpets and slugs.2357

Many are marine, but there is also freshwater gastropods and land-dwelling gastropods.2365

These are mostly herbivores, so they feed on plants. Some are predators.2373

And the radula would be different depending on if it was an herbivore or a predator.2379

What is unique about the gastropods is that they undergo torsion. Torsion is the rotation of the internal organs by 180°.2384

The result is that the GI tract ends up twisted into a U-shape, and that the gills and the anus end up near the anterior end.2403

Many gastropods also have a spiral shell.2411

The next group that we are going to talk about are the Polyplacophora.2416

These are more commonly known as the chitons, and they have a shell consisting of eight plates, so a shell consisting of eight plates.2421

They live in marine habitats and spend their time attached to rocks. They use their foot to attach to the substrate.2431

The next class is the bivalves. Bivalves include clams, oysters, mussels and scallops.2442

These are mostly marine, but there are some freshwater species; and these are a food source for humans.2450

The name bivalve refers to the two parted shell, the two halves that are attached by a dorsal hinge.2455

These do not have a radula, so the bivalves lack a radula.2464

Instead, they are filter feeders, so they do not need a radula, and what they do is they have gills that are coated with mucus.2472

There are cells that secrete mucus that coats the gills, and that allows them to catch food as it filters through.2480

And then, they move the food particles into their mouths using cilia.2486

Bivalves do not have a head, but some of them do have eyes; and others have sensory tentacles.2491

Bivalves are mostly sedentary, but the can move around a little bit using their foot.2497

The next class are the cephalopods. These include squids and octopuses.2503

Cephalopods have a foot that has been modified to form tentacles and a siphon.2509

The siphon is used to squirt water and to move the animal via jet propulsion, so it allows for jet propulsion.2517

What the squid will do is it will pull water into the mantle cavity and then, squirt it out of the siphon, and it can help for movement.2529

The tentacles of cephalopods often have suckers on them.2538

They are different from other molluscs in a few ways. They actually have closed circulatory systems.2542

I said this before where most molluscs have open circulatory systems, so closed circulatory systems.2550

And they have increased cephalization compared to other molluscs.2558

They are predators, and this increased intelligence allows for the behaviors needed to catch prey.2565

They have complex size and a large brain that allows them to hunt and be good predators.2572

Some cephalopods have no shells. These shells were lost during evolution.2583

Some cephalopods have internal shells, and the eyes that cephalopods have are an excellent example of convergent evolution.2588

So, vertebrates have eyes. Cephalopods have eyes, but they evolved to have these eyes separately.2597

And there are many similarities between vertebrate eyes and cephalopod eyes, so we could say these are analogous structures.2604

They evolved separately. There are some differences, as well, though.2611

Arthropod means joint foot, and it includes insects such as bees, crustaceans like crabs and lobsters and other organisms like arachnids and centipedes.2617

Like I said, this is the largest phylum. There are over one million known species of arthropods.2627

And they arose about 500 million years ago during the Cambrian period.2634

They are extremely diverse and are found in virtually every type of climate and habitat.2639

Some live in the ocean. Others are freshwater, and still others are terrestrial.2646

Some arthropods can fly. Others can swim.2650

Some get around by walking.2653

Before we go into the subphyla, we are going to talk about general features of arthropods.2656

They are bilaterally symmetrical, and they have segmented bodies, so their bodies have three regions.2661

They have a head region, a thorax and an abdomen.2667

In some arthropods, the head and the thorax are fused together into what is called a cephalothorax.2675

A second feature in arthropods is the presence of an exoskeleton.2693

An exoskeleton provides protection. It can prevent drying out, provide support and allows a place for attachment of muscles to help them move.2699

The exoskeleton contains chitin, which is the same substance that is found in the cell walls of fungi.2710

Exoskeletons cannot grow. Therefore, if the animal needs to grow, and it has an exoskeleton, it needs to molt or shed it to allow for growth.2720

A third feature, I mentioned the name means joint foot, and arthropods have paired jointed appendages.2729

They have an open circulatory system containing a fluid called hemolymph that contains nutrients, delivers oxygen and hormones.2742

The hemolymph will go through arteries. It will go through blood vessels and then, just enter cavities around the organs.2752

The excretory system consists of malpighian tubules that remove waste products.2762

The respiratory system depends on the environment that the arthropod lives in.2775

Some arthropods have gills. Those that live on land have trachea, so it could be gills, trachea, which are tubes or air ducts to bring air in.2782

Other groups such as arachnids have what is called book lungs.2796

And book lungs, they look like a bunch of paper folded up or stacked up like a book.2800

And that allows for a larger surface area across, which gas exchange can occur.2806

The nervous system of arthropods is fairly well-developed.2811

They have sensory organs such as eyes, olfactory organs for smell, antennas for touch, and some species even have ears.2815

Some species have ganglia. Others have brains, and those are the generalities.2823

And now, we are going to talk about some of the subphylums of these group because it is such a large group.2829

The first ones we are going to talk about are the Myriapoda. Myriapoda include millipedes and centipedes, and these both live on land.2836

They also have jaws. They have mandibles.2854

Centipedes have one pair of legs per segment, whereas, millipedes have two pairs of legs per segment.2856

Another difference between the two is that centipedes have longer antennas than millipedes.2869

And centipedes are predators, whereas, millipedes are primarily scavengers.2875

So centipedes are predators. Millipedes are scavengers.2882

Since centipedes are predators, the front set of their legs are modified and contain toxins that will kill their prey.2891

The next subphylum are the chelicera forms, and the chelicera forms mostly consist of arachnids, so that is what I am going to focus on,2900

so mostly arachnids in this group such as spiders, ticks, scorpions.2915

And looking at spiders, they have a cephalothorax and an abdomen. They have four pairs of legs, and they also have a pair of chelicerae.2931

Chelicerae are pinchers or fangs that are located near the mouth that aid in feeding, so four pairs of legs plus one pair of chelicerae.2941

Arachnids lack antenna and mandibles.2960

Some like spiders are predators. Others like ticks are parasites.2963

So, this is Myriapoda's one subphylum, two, and the third subphylum that we are going to talk about are the crustaceans, so, subphylum Crustacea.2968

These include crabs, lobsters, shrimp, and many of these are found in marine habitats.2982

There are also freshwater habitats, and some members of this group are terrestrial.2990

They have two pairs of antennas and two sections to their body, the cephalothorax and the abdomen, and gas exchange occurs through gills.2994

The next subphylum is Hexapoda.3004

Hexa means six, and this group includes insects such as butterflies, bees, crickets and hundreds of thousands of other species, so six legs.3011

So, the thorax is divided into three segments each with a pair of legs.3025

Some hexapods also have wings attached to the thorax, and these wings evolved independently of bird wings and of bat wings.3029

Bird wings and bat wings are both modified for limbs. They are modified appendages.3039

Whereas, the wings that are attached to the thorax are separate, evolved independently, have a completely different structure.3044

They have a cerebral ganglion at the head end and compound eyes, so Hexapoda have compound eyes.3053

What that means is that their eyes are made up of many visual receptors each with their own lens.3064

Another interesting thing about this group is that they exhibit metamorphosis, and there are two types of metamorphosis.3071

There is gradual or incomplete and there is also complete, so metamorphosis can be gradual or complete.3080

In gradual metamorphosis, it is less dramatic. For example, grasshoppers go through gradual metamorphosis.3093

So, that involves going from an egg to a nymph to an adult.3102

And the nymph is not so dramatically different form the adult, although, the nymphs do not have wings.3110

Complete metamorphosis, which is what we often think of because that is what butterflies go through, is more dramatic.3118

In this type of metamorphosis, the insect goes from egg to the larva stage to pupa and then, finally, to the adult stage.3130

The caterpillar is the larva stage in a butterfly, and as you know, it looks very different from the adult butterfly.3144

The larva in a butterfly is a caterpillar. The pupa, the resting stage, is the chrysalis, and then, they turn into the adult butterfly.3153

Reproduction in hexapods is sexual, and that is the last subphylum except there is one additional one, but all the members of that are extinct.3162

And we are going to stick to talking about subphylum that have organisms still in existence.3173

The last phylum we are going to talk about is the echinoderms, phylum Echinodermata, and echinoderm means spiny skin.3180

Members of this group include sea urchins, sea stars and sand dollars.3194

Sometimes these are called starfish, but they are not fish either; so the correct name is sea stars.3200

These are the first deuterostomes that we are talking about.3205

And you remember from a previous lecture, we talked about that in embryological development, a deuterostome has radial determinate cleavage.3211

We will talk more about this in the lecture on reproduction and embryology.3222

Some members of this group are sessile. Others are motile but slow moving.3233

As embryos, they are bilaterally symmetrical, but as adults, they are mostly radially symmetrical.3237

And I say mostly because they sometimes have features that are not perfectly radially symmetrical.3244

They have a calcium-containing endoskeleton, and the spikes on this give them their bumpy appearance and provide protection for them.3251

They also have a water vascular system, and that is a part of what is shown below in the sea star.3260

This is a system of canals - in the green - that radiates out from a central ring, so there is a central ring canal.3270

And what happens is water enters and leaves the system through an opening called a madreporite, so water enters the system3281

and then, goes to the ring canal, down through radial canals, so the ring canal and then, to the radial canals, then, into lateral canals.3291

And these lateral canals end in ampullas.3308

The ampullas lead to tube feet, so the starfish has tube feet, and actually, the ampulla pushes water into the podium portion of the tube feet.3314

So, when water enters, it pushes water into the tube feet.3328

That causes the tube feet to expand, and then, they will touch the substrate and attach via suckers.3339

When water is pushed back up into the ampulla, the feet will contract and the suckers will release from the substrate.3346

So, water enters, the starfish can attach to a substrate. Water leaves the ampulla, it will release the substrate.3355

Tube feet are also used by a sea star to grab prey.3365

The mouth is often located on the downward facing side of a sea star, and it leads to a stomach, so the stomach is here in purple, then, to an intestine.3369

And waste is eliminated through an anus, which is often on the opposite side of the sea star from the mouth.3383

The nervous system in this phylum is not well-developed. There is a nerve ring and chords that radiate out into the arm.3389

Gas exchange is through gills on the surface.3397

If you were to go deeper into classification, sea stars and what is called brittle stars have some similarities, but they are actually in different classes.3403

If you look at them, brittle stars are, sort of, more delicate looking. They have longer, thinner arms.3413

Sea urchins, sea cucumbers and sand dollars do not have arms at all.3419

OK, the trend overall, as we discussed the various phyla, was going from more simple to more complex.3424

And just to sum up some characteristics of invertebrates, on the left is listed the different phylum,3432

and here, whether it is a diploblastic or a triploblastic group, so does it have two cell layers or three.3439

Symmetry: Porifera had no symmetry. The cnidarians are radially symmetrical, and then, we moved on to the flatworms.3446

These are bilaterally symmetrical as were all the other groups we covered except the echinoderms,3455

which we just covered that are bilaterally symmetric during development but then, later on are somewhat radially symmetrical.3465

I included Chordata here because there are some groups of Chordata that are invertebrates.3473

Then, most of them, the primitive groups...so, I did not even divide these two into protostomes,3480

deuterostomes because you can only make that division if there are three layers of tissue.3486

So, we had protostomes, protostome and so on until we got to the echinoderms, so those were our first deuterostomes.3492

Porifera and Cnidaria as well as the flatworms lack a coelom, then, we got to the rotifers and the nematodes that have a pseudocoelom,3500

and then, finally, annelids, which have a true coelom.3509

We also talked a lot about organ systems and how have those evolved to become more complex.3513

And we will talk more about physiology in a separate lecture.3519

Example one: state the name of the phylum that fits each of the descriptions below.3524

Have a polyp or medusa body plan- so, we talked about a phylum that contains a polyp or medusa body plan.3530

And an example would be the jellies, which are mostly found in a medusa form, and these are members of the phylum Cnidaria.3538

Bodies have a head foot region, a visceral mass and a mantle that may secrete a shell- so, this is a large diverse phylum including snails, slugs and squids.3552

This is phylum Mollusca.3563

Members are free living or parasitic unsegmented worms with a complete GI tract and a pseudocoelom-3566

so, these are unsegmented worms, yet, they do have a complete GI tract, and they have a pseudocoelom. These are actually the nematodes- Nematoda.3574

OK, so that was example one.3588

Example two: discuss two ways in which animals have evolved to become more complex.3590

There is many more than two.3596

But, one that you could list could be symmetry, that groups of animals range from no body symmetry to radial symmetry to bilateral symmetry3598

going from two germ layers, ectoderm and endoderm to three germ layers, ectoderm mesoderm and endoderm.3616

The simplest animals, the sponges, do not have tissues and organs, so no tissues or organs to developing tissues or organs systems.3632

They are very complex.3643

Cephalization: very simple animals do not have a head end. More complex animals have a head end where sensory organs are concentrated.3652

Example three: match the following organisms to the correct phylum.3664

Sea stars: sea stars were one of the ones we just finished discussing, and remember that those are members of the phylum Echinodermata.3669

Crickets: well, crickets are insects, and they are members of Arthropoda.3682

Leeches: leeches are segmented worms, and they belong to phylum Annelida.3691

Four, sponges: sponges are among the simplest animals, and they belong to phylum Porifera.3701

And then, finally, tapeworms belong to the group Platyhelminthes.3710

Describe the general characteristics of the phylum Arthropoda, so there are multiple subphyla.3722

We are just going to talk about some general features that you might find in arthropods and that is that they are bilaterally symmetrical.3728

They have three regions, so they are segmented.3743

They have three regions. They have a head region, a thorax and an abdomen.3748

They may have an exoskeleton that provides support and protection. They have paired jointed appendages.3757

OK, so that completes this section on the invertebrates.3777

Thank you for visiting Educator.com.3781