FOR THOUSANDS of years, people have sought to

Transcription

1 Classifying Animals FOR THOUSANDS of years, people have sought to classify living creatures into general categories. An ongoing process, classification aids in research, education, and communication about different species. Newly discovered animals, insects, and plants are added to the categories each year. Objective: Identify and describe the six most important classes of animals. Key Terms: amphibians arthropods cold-blooded exoskeleton invertebrates mammals vertebrates warm-blooded Classes of Animals Throughout the world, millions of animals exist, and there are many classes in which to place them. Every animal is classified based on its characteristics. One of the most general classifications is based on whether the animal has a spine or a backbone. Vertebrates are animals that possess a backbone, while invertebrates are animals that do not possess a backbone. It may seem as if there are more vertebrate animals, but invertebrates, specifically insects, outnumber us. Page 1

2 Animals also can be classified by the way they control their body heat. Warm-blooded animals can regulate their own body heat. As a result, they have adapted to a variety of environments. In contrast, cold-blooded animals must rely on the temperature of their environment to heat or cool their bodies. For this reason, they cannot survive in places with extremely hot or cold temperatures. SIX MAJOR CLASSES OF ANIMALS Most animals on Earth are grouped into six major classes: mammals, birds, fish, reptiles, amphibians, and arthropods. Mammals Mammals are warm-blooded animals that have hair, give birth to live young, and produce milk (from a mammary system) for their young. Mammals are vertebrates and include most companion animals, common livestock, and humans. Birds Birds are warm-blooded animals, but they hatch from hard-shelled eggs. Birds are the only animals that have feathers, FIGURE 1. These cattle are warm-blooded vertebrates. FURTHER EXPLORATION Online Connection: Researching Animal Classes The task of classifying all animals into a logical class system is daunting. Scientists are continually updating, revising, and adding new life forms as they are discovered. The Internet has dramatically affected the way scientists can share information about animal classes and educate others on the subject. Although it would be nearly impossible to list every animal species on one concise Web site, many sites are wonderful resources when researching each of the major animal classes. To explore the major animal classes further, visit the link below: Page 2

3 but not all birds can fly. Eagles, robins, chickens, and penguins are all examples of birds. Fish Fish live their entire lives in water by using a set of gills to extract oxygen from the water to breathe. Normally fish have scales covering their bodies. For reproduction, female fish lay eggs that are then fertilized by the male in a process called spawning. Examples of fish include bass, bluegill, trout, and tilapia. FIGURE 2. Like most other birds, eagles have the ability to fly. FIGURE 3. Fish are generally covered with scales and live in water at all times. Reptiles Reptiles are cold-blooded animals that are born on land, usually from eggs. Reptiles normally have scaly skin and breathe air with a set of lungs. Common reptiles are turtles, alligators, snakes, and lizards. FIGURE 4. Snakes are cold-blooded animals that give birth by laying eggs. Page 3

4 Amphibians Amphibians are animals that are born in water. These animals use gills to breathe and later form lungs that allow them to travel on land and to breathe air. Amphibians are cold-blooded and often undergo radical physical changes when growing to adulthood. Toads, frogs, salamanders, and newts are all amphibians. Arthropods Arthropods generally are animals that have more than two pairs of jointed legs, such as arachnids (spiders), insects, and crustaceans. Arthropods commonly have an exoskeleton (a hard, outer covering that serves the function of an interior skeleton). All insects and spiders as well as lobsters, crabs, and shrimp are included in the arthropod class. FIGURE 5. Although frogs are born in water, they will eventually undergo dramatic changes and will be able to travel on land. Summary: FIGURE 6. Arthropods, such as this grasshopper, have an outer covering known as an exoskeleton. All animals can be classified into groups based on their characteristics. The six most important classes of animals are mammals, birds, fish, reptiles, amphibians, and arthropods. Mammals are animals that give milk, have hair, and give birth to live young. Birds are the only group with feathers, and they have young that hatch from hard-shelled eggs. Fish are cold-blooded and use a set of gills to breathe underwater. Although reptiles are also cold-blooded, they live on land and breathe air. Amphibians begin life underwater using gills, but they form lungs and breathe air as adults. Finally, arthropods are the largest group and include insects, arachnids, and crustaceans. Page 4

5 By classifying animals, we make it easier to study their characteristics and learn more about how they interact with the environment and, ultimately, all of us. Checking Your Knowledge: 1. Explain the differences between vertebrates and invertebrates. 2. List the six animal groups, and give two examples of animals from each group. 3. How are amphibians different from reptiles? 4. Why are arthropods the largest class of animals? 5. Provide three characteristics of mammals. Expanding Your Knowledge: Imagine you are a scientist employed to classify new animals. Create your own key with steps to follow in placing animals in the six major categories. Model your key to look like a dichotomous key. Web Links: Classes of Animals The Use of Dichotomous Keys Agricultural Career Profiles Page 5