Chapter 5: Vertebrate Animals 157

VOCABULARYvertebrate p. 157endoskeleton p. 157scale p. 161

BEFORE, you learned

• Most animals are invertebrates• Animals have adaptations that

suit their environment• Animals get energy by

consuming food

NOW, you will learn

• About the skeletons of vertebrate animals

• About the characteristics of fish

• About three groups of fish

KEY CONCEPT

Vertebrates are animalswith endoskeletons.

Vertebrate animals have backbones.If you asked someone to name an animal, he or she would probablyname a vertebrate. Fish, frogs, snakes, birds, dogs, and humans are all or animals with backbones. Even though only about 5 percent of animal species are vertebrates, they are among the mostfamiliar and thoroughly studied organisms on Earth.

Vertebrate animals have muscles, a digestive system, a respiratorysystem, a circulatory system, and a nervous system with sensoryorgans. The characteristic that distinguishes vertebrates from otheranimals is the an internal support system that growsalong with the animal. Endoskeletons allow more flexibility and waysof moving than exoskeletons do.

endoskeleton,

vertebrates,

EXPLORE Streamlined Shapes

How does a fish’s shape help it move?

PROCEDURE

Place your hand straight up and down in a tub of water. Keep your fingers together and your palm flat.

Move your hand from one side of the tub to the other, using your palm to push the water.

Move your hand across the tub again, this time using the edge of your hand as if you were cutting the water.

3

2

1

MATERIALStub of water

WHAT DO YOU THINK?• In which position was the shape of your

hand most like the shape of a fish’s body?• How might the shape of a fish’s body

affect its ability to move through water?

VOCABULARYAdd magnet word diagrams for vertebrateand endoskeleton to your notebook.

158 Unit 1: Diversity of Living Things

Vertebrates are named for specialized bones called vertebrae. Thesebones are located in the middle of each vertebrate animal’s centralbody segment. Together, the vertebrae are sometimes called a back-bone. The vertebrae support muscles and surround the spinal cord,which connects the animal’s brain to its nerves. Other bones, such asthe ribs and skull, protect organs like the heart, lungs, and brain.

Check Your Reading What is one function of the endoskeleton for vertebrate animals?

Most vertebrates are fish.Fish are the most diverse group of vertebrate animals. There are morethan 20,000 species of fish, ranging in size from tiny minnows to hugewhale sharks. Fish live in nearly every aquatic environment, fromfreshwater lakes to the bottom of the sea. Some fish even are able tosurvive below the ice in the Antarctic!

Fish are adapted for life in water. Like all living things, fish need toget materials from their environment. For example, fish must be ableto get oxygen from water. Fish must also be able to move throughwater in order to find food. Fish that live in water where sunlight doesnot penetrate need special organs to help them find food.

Most fish move by using muscles and fins to push their stream-lined bodies through water. These muscles allow fish to move more

NOTETAKING STRATEGYChoose a strategy from an earlier chapter to takenotes on the idea that mostvertebrates are fish. Be sureto include information onadaptations to water.

Vertebrate Skeleton and Muscles

ribs

Muscles attachedto bones aid inmovement.

Like all vertebrates, the cheetah has an internal skeleton, which supports and protects its body.

muscle

vertebrae(backbone)

skull

leg bones

How does this cheetah’s endoskeletonrelate to the cheetah’s shape?

Chapter 5: Vertebrate Animals 159

quickly than most other vertebrates. Most fish also have an organcalled a swim bladder, which allows them to control the depth atwhich they float.

Fish have sensory organs for taste, odor, and sound. Most fishspecies have eyes that allow them to see well underwater. Most fish alsohave a sensory system unlike that of other vertebrates. This systemincludes an organ called a lateral line, which allows fish to sense vibrations from objects nearby without touching or seeing them.

Fish, like some invertebrates, remove oxygen from water with spe-cialized respiratory organs called gills. You can locate most fishes’ gillsby looking for the openings, called gill slits, on the sides of their head.You can see what gills look like in the diagram of a fish below.

Check Your Reading How are gills similar to lungs?

Fish gills are made up of many folds of tissue and are filled withblood. When a fish swims, it takes water in through its mouth andthen pushes the water back over its gills. In the gills, oxygen dissolvedin the water moves into the fish’s blood. Carbon dioxide, a wasteproduct of respiration, moves from the blood into the water. Thenthe water is forced out of the fish’s body through its gill slits. Theoxygen is transported to the fish’s cells. It is a necessary material forreleasing energy.

VISUALIZATIONCLASSZONE.COM

Explore how fishbreathe.

Inside a Fish

What is the function of the vertebrae on the fish’s backbone?

Swim bladder allows the fish to adjust how high or low it floats.

Lateral line is an organ inthe sensory system that allowsfish to sense objects and organ-isms that are nearby.

Gills remove oxygenfrom water and exchangeit for carbon dioxide inthe fish’s blood.

Fish are vertebrates that live in water.

Vertebrae make up part ofthe endoskeleton of this fish.

160 Unit 1: Diversity of Living Things

Fish can be classified in three groups.Scientists classify fish into three major groups: jawless fish, cartilagi-nous fish, and bony fish. Each group is characterized by body features.As you read on and learn about each group of fish, look at the pho-tographs above.

Jawless FishScientists think that fish in this group, which includes lampreys andhagfish, are the living animals most similar to the first fish thatlived on Earth. Jawless fish have simpler bodies than the other fish.They have a slender tubelike shape and a digestive system without a stomach.

As the name of the group implies, these fish do not have jawbones. Although they do have teeth, they cannot chew. Most jawlessfish eat by biting into another animal’s body and then sucking outflesh and fluids.

Check Your Reading What is a characteristic of jawless fish?

Scientists usually classify fish in three groups.

Three Groups of Fish

What similarities do you notice about the bodies of the fish in the photographs?

Cartilaginous Fish

Cartilaginous fish like this hammerhead shark have skeletons made of cartilagerather than bone.

Jawless fish like the lamprey on the left and the hagfish on the righthave tube-shaped bodies.

Jawless Fish Bony Fish

Most fish living now, includingthis queen angelfish, have bonyskeletons and are covered with scales.

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Learn more about fish.

Chapter 5: Vertebrate Animals 161

Cartilaginous FishThis group includes sharks, rays, and skates. Their skeletons are notmade of hard bone, but of a flexible tissue called cartilage (KAHR-tuhl-ihj). Some species of sharks are dangerous to humans, but mostcartilaginous fish feed primarily on small animals such as mollusksand crustaceans. Whale sharks and basking sharks, which are thelargest fish on Earth, feed by filtering small organisms from the wateras they swim.

Rays are flat-bodied cartilaginous fish that live most of their liveson the ocean floor. Their mouths are on the underside of their bodies.Most rays eat by pulling small animals out of the sand. A ray’s flatbody has fins that extend on either side of its vertebrae like wings.When rays swim, these fins wave so it looks as if the fish is flyingthrough the water.

Check Your Reading Describe three ways cartilaginous fish species obtain food.

Bony FishMost fish species, including tuna, flounder, goldfish, and eels, are clas-sified in this large, diverse group. Of the nearly 20,000 fish species,about 96 percent are bony fish. Bony fish have skeletons made of hardbone, much like the skeleton in your body. Most bony fish are coveredwith overlapping bony structures called They have jaws andteeth and several pairs of fins.

The range of body shapes and behavior in bony fish shows howliving things are adapted to their environments. Think of the brightcolors and patterns of tropical fish in an aquarium. These eye-catch-ing features are probably adaptations for survival in the fishes’ naturalenvironment. In a coral reef, for example, bright stripes and spotsmight provide camouflage or might advertise the fish’s presence toother animals, including potential mates.

Most young fish develop inside an egg.Most fish species reproduce sexually. The female produces eggs, andthe male produces sperm. In many fish species, individual animalsselect a mate. For example, a female fish might release eggs into thewater only when a certain male can fertilize them. After the eggs arefertilized, the parent fish usually leave the eggs to develop and hatchon their own. Most fish reproduce this way, but there are many exceptions.

scales.

reminder

In sexual reproduction thegenetic material from twoparents is combined in theiroffspring.

All animals and plants that can reproduce sexually produce eggcells and sperm cells. However, the structure and size of eggs varyamong species. In Chapter 3, you learned that the eggs of floweringplants are found inside the seeds in fruit. In Chapter 4, you read aboutthe eggs of different types of invertebrates. You learned that some eggshave a food supply and a protective covering. For some animals, thefood supply is called yolk, and the covering is called an egg case.

Most fish eggs are surrounded by a soft egg case that water canpass through. Since fish lay eggs in the water, this means that a fishembryo inside an egg gets the water and oxygen it needs directly fromits surroundings. The egg’s yolk provides the developing fish withfood. Such eggs can develop on their own, without needing care fromadults. However, many animals eat fish eggs. Fish often lay and fertil-ize many eggs, but few of them survive to maturity.

Check Your Reading How are fish eggs different from invertebrate eggs?

KEY CONCEPTS1. Why are fish classified as ver-

tebrate animals?

2. What are three adaptationsthat suit fish for life in water?

3. Name a feature for fish fromeach of these groups: jaw-less, cartilaginous, and bony.

CRITICAL THINKING4. Apply If you wanted to

choose tropical fish that couldlive comfortably in the sametank, what body features orbehaviors might you look for?

5. Infer Some fish do not layeggs. Their eggs develop insidethe female fish. How mightthe offspring of one of thesefish differ from those of anegg-laying fish?

CHALLENGE6. Synthesize Fossils indicate

that species of fish with bodiesvery similar to today’s sharkshave lived in aquatic environ-ments for hundreds of millionsof years. What can you inferabout the adaptations ofsharks from this?

162 Unit 1: Diversity of Living Things

This young shark is devel-oping from an egg that iscovered by an egg case. In this photograph, lightshining through the eggcase allows you to see theshark and the yolk inside.

reminder

An embryo is the imma-ture form of an organismthat has the potential togrow to maturity.

yolk

developing shark

egg case