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  Nature Elective

The Five Classes of Vertebrates
First posted June 12, 2004 Last updated November 20, 2009


Everyone loves animals, and almost all the animals you know are vertebrates.

Can you name some animals? Write down a list of as many animals as you can. If you look over this list, probably all of the animals that you have named have bones, with a series of large bones in their backs that helps them to move (run, climb, fly, swim). This is called their backbone or vertebral column. All animals with a backbone are called vertebrates. Do you have a backbone? Then you are a vertebrate, too! Vertebrates make up most of the "animals" that we are familiar with. Exceptions to this would be things like an insect, an octopus, a lobster or crab, a jellyfish, or a clam. Most everything else is a vertebrate, with a backbone and other bones.

Vertebrates can be classified into five groups, based on their skin covering, how they reproduce, how they maintain body temperature, and characteristics of their limbs (arms and legs, or their equivalent such as wings or fins). Knowing the five classes of vertebrates and their characteristics helps you to understand more about these animals.

The five classes of vertebrates are listed below.

You probably know all of the characteristics that are in bold. If you know this much, you know enough to classify any animal you see! Learn these characteristics if you don't already know them. Just think of your favorite example and the characteristics become very easy to remember. The other information gives you more characteristics that scientists use to classify animals into their respective class.


Coho Salmon

1. Fish are ectothermic, aquatic vertebrates.
2. Their skin is generally covered with scales.
3. Their limbs are modified into fins for swimming.
4. They breathe with gills.
5. They lay eggs that must be in water.

You probably knew most of this already, didn't you? The term "ectothermic" may be new to you. It means "external temperature" in Latin. The simpler term is "cold-blooded", that is, it has a body temperature equal to the surrounding environment. You know that fish are cold-blooded if you ever picked one up: they are as cold as the water they were swimming in. Even if you go swimming and are shivering with cold, you are always warmer than the water. "Aquatic" means that they spend their lives in the water.
The most prominent fish at Cazadero is a visitor during the winter rains: the Coho salmon. A federally threatened species, this ocean-dwelling fish comes up Austin Creek when the winter rains raise the water level, to spawn (lay eggs) in the gravel beds that are so typical of the redwood creeks. Note that the eggs are laid in the water, not on land (reptiles and birds lay eggs on dry land).


  The eggs drift down into the gravel, where they are protected. The eggs at the left are Coho salmon eggs, and they show the developing embryo, and you can see the eye of the baby fish.

A salmon is a fish because it is ectothermic (cold-blooded), lives in the water, breathes with gills, lays eggs in water, and has fins and scales.







Pacific Treefrog


1. Amphibians are ectothermic vertebrates.
2. Their skin lacks scales, hair, and feathers, and is either smooth (like a frog) or rough (like a toad). They are dependent upon moisture and subject to desiccation; their skin must remain moist to aid in breathing.
3. They lay eggs in water, which hatch into an intermediate life form (tadpole or larva) that usually breathes with gills, and change into the adult form that breathes air and can live outside water.
4. They have three-chambered hearts.
5. They lack claws on their toes.

You already knew all the facts that are in bold, didn't you? Let's think of the various kinds of amphibians that Cazadero has, and you can probably figure out all the characteristics of the class. The frog at the top is the Pacific treefrog, one of the most famous frogs in the world. Do you know why? It is the frog heard round the world! Most Hollywood movies have used the croaking of Pacific treefrogs when they want frog noises, because they are champion croakers. They are treefrogs, so they do not spend much of their time in water, but are found in trees.

The toad (below, left) is the Western Toad, which you can find in the thousands down by Austin Creek in later summer when they have hatched out from their tadpole stage. The amphibian (below, right) is the California Newt. Newts are like salamanders, but they have the rougher, dryer skin like a toad. Frogs are like salamanders, and toads are like newts. They all have moist skins, through which they can breathe (they also have lungs for breathing).The salamander at the bottom is the California Slender Salamander, which is about 3 inches long but only 1/4 inch wide.

The name "amphibian" comes from the Latin for "two lives", which refers to the fact that all amphibians have a larval stage (we call the larval stage of frogs and toads a "tadpole" or "pollywog") and an adult stage. This is unique in the vertebrate world, because fish, reptiles, birds, and mammals are all born as small versions of the adult, not a completely different form. This dual life, one aquatic with gills and one terrestrial with lungs, may be representative of the evolution of animals out of water and onto dry land.

Now, lets look at the characteristics of the class amphibian. You already knew that frogs were cold-blooded if you ever picked one up. You also knew that tadpoles hatched into frogs, and that frogs do not have feathers, scales, or hair ("fine as frog fur" is a famous phrase), and that their skin was moist. You probably already knew that frogs lay eggs. So you are already an expert on what makes an amphibian an amphibian! The other characteristics may be fun to know, but you can classify an animal as an amphibian just by what you already know.

Frogs, toads, salamanders, and newts are all amphibians because they are cold-blooded, have no scales, hair, or feathers, their skin is moist, they lay eggs in water, and their life cycle has two stages, the "pollywog" or larval stage and the adult stage.


Garter Snake
(click here for larger image)

1. Reptiles are ectothermic vertebrates.
2. Their skin has scales, but no hair or feathers.
3. They have three-chambered hearts (except for alligators and crocodiles, which have four-chambered hearts).
4. They have claws on their toes (except those which do not have legs, such as legless lizards).
5. They are the first animals, in evolution, to develop the amniotic egg. This allows reptiles to lay eggs on land.

You already knew all the facts that are in bold, didn't you? Let's think of the various kinds of reptiles at Cazadero, and review what we already know about them.

The most common reptile that you can find at Cazadero is the alligator lizard, usually crawling though the leaves and debris on the ground, looking for invertebrates to munch and lunch on. You may also see garter snakes along the creek side, blue belly lizards sunning themselves on rocks or logs, or the rare turtle in the creek.

What do all these reptiles have in common? They all are cold-blooded (the reason lizards and snakes sun themselves is to warm up, just like you on a cold morning, or after a wade in Austin Creek) and they have scales. Did you ever see a lizard with feathers, or a hairy snake? No! Scales make up their only covering. Reptiles also lay eggs on dry land. You have heard of turtle eggs, right? Nature shows often have programs on sea turtles and how they lay their eggs on the beach. You may have seen some nature programs on alligators, another kind of reptile but not one we have, laying eggs in piles of rotting vegetation. All reptiles lay eggs. We know now that birds evolved from reptiles, and one of their linkages is the fact that they both lay hard-shelled eggs on dry land. Amphibian eggs, in comparison, are soft and must be laid in water.

(Photo by DLNelson, at camp, 2008)


Snakes, turtles, and lizards are all reptiles because they are cold-blooded, they lay eggs on dry land, and are covered with scales, never feathers or fur.


1. Birds are endothermic vertebrates.
2. Their skin is covered with feathers.
3. They have four-chambered hearts.
4. Their bones are lightweight and usually hollow.
5. Their forelimbs are modified as wings.
6. They lay eggs.

You already knew all the facts that are in bold, didn't you? Everyone knows birds have wings with feathers and lay eggs. You may not have known they are endothermic, which is Latin for "inside temperature". This means that they can maintain their own warm inside temperature, even when it is cold outside. Frogs (amphibians) and snakes (reptiles) have to hide when the weather is cold, but birds can be out and active. Let's think of the various kinds of birds at Cazadero, and think what we already know about them.

The most common birds at Cazadero are blue jays, both Scrub Jays and Steller's Jays (photo above), robins (at right), as well as a variety of aquatic birds such as ducks, mergansers, and herons, which feed along Austin Creek.

The characteristics of the class Birds are rather well-known and hardly need to be taught. We all know that birds have wings, chickens lay eggs, and that birds of a feather flock together. The only character that you might not have known is that they are warm-blooded, unless you or a friend have a pet bird at home or if you have held a chicken. Birds in general have a higher body temperature than we do, so they feel quite warm when we hold them. Their feathers are well-adapted to hold this heat in, which is why we make comforters out of down.


Jays, robins, and ducks are all birds because they are warm-blooded, have feathers, wings, and lay eggs.


(Photo by DLNelson, November 9, 2009, but not at camp)

1. Mammals are endothermic vertebrates.
2. They have hair, which varies greatly among species.
3. Most have sudoriferus (sweat) glands.
4. They have mammary (milk-secreting) glands.
5. They have sebaceous (fat-secreting) glands.
6. They have heterodont dentition (different types of teeth).

Cazadero has a lot of mammals, but they tend to live elsewhere and only move through the Redwood Forest. There is not much in the way of natural food in the redwoods, but camp provides a lot of human food, so raccoons and mice are frequent guests. The opossum and deer might wander along Austin Creek.
Deer, mice, opossums, raccoons, and humans are mammals because they are warm-blooded, have hair, and nourish their young on milk.



How about a test of your knowledge? This will also show you the usefulness of knowing the classification of vertebrates.

Animal One
You come to a strange island and the islanders tell you about a strange creature that has a backbone. They say it flies around with two wings. What is your first guess? You would probably say a bird. But they tell you it does not lay eggs. Could not possibly be a bird. It has fur. Can you now guess? Answer here.

Animal Two
You are also told about an animal with a backbone that is now extinct, but the elders remember that it ran around on the ground and couldn't fly like a bird. But it laid eggs. What is your first guess? You would probably say a reptile, like a lizard. But they tell you it did not have scales, but feathers. Can you guess what it must be? Answer here.

Animal Three
You are also told about an animal with a backbone with no legs that lays eggs, but is definitely not a snake. What is your first guess? You would probably say a fish. However, this strange creature lives on land, has scales, and lays eggs on the dry land. Answer here.

Animal Four
The islanders also tell you about an animal that lives its whole life in the water and has gills. What is your first guess? You are wrong! It is not a fish! They tell you that it lays eggs in the water, does not have a larval stage but is born as a small version of an adult of the species. Guess again? It can't be an amphibian, as it does not have a larval stage. Reptile? No, it has gills. Reptiles breathe with lungs. Mystified? Carefully check the description and click here.

Animal Five
The final animal that you are told about is rather strange, indeed. It has a backbone, swims in the water and lays eggs. What is your first guess? You might think that it is a fish or an amphibian, or even a reptile or bird. They all lay eggs and some of each group swims. You really can't guess yet. They tell you it has claws and a bill like a duck. You might think it is a bird. Then they tell you it has no feathers. That rules out a bird. What's left? A fish, an amphibian, or a reptile, but this is strange. A fish with a bill like a duck? Then they tell you it breathes air, and its eggs are laid on the ground. That rules out what? A fish. Reptile? But they tell you it is warm blooded, and nurses its young with milk. Boy, you are in a quandary now! What class would a vertebrate be in? Answer here.


Reptilia's webpage on teaching the five classes to fifth graders is nice.

Smithsonian's Vertebrate webpage is for fairly advanced students.

The WebQuest page on vertebrates is an interactive learning tool and quite nice.


The photo of Coho eggs is from a newsletter of Kalles Junior High.

The Western Toad and California Newt are from Richard Paslek's webpage on the Humbolt State University website (interestingly, in the Chemistry Department's section!). The California Slender Salamander photograph was taken by Jason Lowe.

The garter snake photo is from Gerry M. Serianni's webpage on his geneology.

The Steller's Jay and robin are from a webpage on insect-eating birds posted by the Federal government.

The deer mouse, opossum, and Pallid bat are from the Math/Science website; this group is located in Fremont.

The dodo bird is from the WebQuest site, cited under References. The legless lizard is from the Applied Ecology Research Group: Fauna Notes.